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Joint Geological Survey/University of

MARINE GEOSCIENCE UNIT

TECHNICAL ^REPORT NO. 13

PROGRESS REPORTS FOR THE YEARS 1981-1982

Marine Geoscience Group Department of Geology University of Cape Town December 1982 NGU-Tfc—Kh

JOINT GEOLOGICAL SURVEY/UNIVERSITY OF CAPE TOWN

MARINE GEOSCIENCE UNIT

TECHNICAL REPORT NO. 13

PROGRESS REPORTS FOR THE YEARS 1981-1982

Marine Geoscience Group Department of Geology University of Cape Town December 1982 The Joint Geological Survey/University of Cape Town Marine Geoscience Unit is jointly funded by the two parent organizations to promote marine geoscientific activity in South . The Geological Survey Director, Mr L.N.J. Engelbrecht, and the University Research Committee are thanked for their continued generous financial and technical support for this work. The Unit was established in 1975 by the amalgamation of the Marine Geology Programme (funded by SANCOR until 1972) and the Marine Geophysical Unit. Financial ?nd technical assistance from the South African National Committee for Oceanographic Research, and the National Research Institute for Oceanology (Stellenbosch) are also gratefully acknowledged. It is the policy of the Geological Survey and the University of Cape Town that the data obtained may be presented in the form of theses for higher degrees and that completed projects shall be published without delay in appropriate media. The data and conclusions contained in this report are made available for the information of the international scientific community with tl~e request that they be not published in any manner without written permission. CONTENTS

Page INTRODUCTION by R.V.Dingle i

PRELIMINARY REPORT ON THE BATHYMETRY OF PART OF 1 THE TRANSKEI BASIN by S.H. Robson DISTRIBUTION OF BENTKIC OSTRACODS DURING THE 6 EMRLY SEPARATION OF AND THE FALKLAND PLATEAU by R.V. Dingle PRELIMINARY REPORT ON GEOCHEMICAL STUDIES OF 17 TUGELA RIVER MUDS CM THE CONTINENTAL SHELF by T. Felhaber USING THE SCANNING ELECTRON MICROSCOPE AND 19 ENERGY-DISPERSIVE X-RAY SPECTROMETER TO DO MINERAL IDENTIFICATION AND COMPOSITIONAL POINT COUNTING ON UNCONSOLIDATED MARINE SEDIMENTS by S.H. Robson (Deleted) BASEMENT MORPHOLOGY AND UNCONSOLIDATED SEDIMENT 28 IN by J.M. 3renner and A. du Plessis

GEOPHYSICAL INVESTIGATIONS IN l-ti*: VICINITY OF 37 HARBOUR by J.M. Bremner THE , SOUTH ATLANTIC OPENING AND 51 RIDGE-JUMPS SOUTH OF THE AGULHAS FALKLAND FRACTURE by A.K. Martin, C.J.H. Hartnady and D.B.Murray BATHYMETRY OF THE INNER SHELF ALONG THE CAPE WEST 6 4 COAST BETWEEN THE AND PORT NOLLOTH by R.H. De Decker QUATERNARY SEDIMENTATION OF THE BOT RIVER LAGOON 72 by J. Rogers BEACH MORPHODYNAMICS IN RELATIONSHIP TO WAVE 97 ENERGY, GRAIN SIZE AND INTERNAL SEDIMENTARY STRUCTURE by B.W. Flemming PRELIMINARY MODEL OF SEDIMENT DISPERSAL BETWEEN 106 PORT ST JOHNS AND THE MSIKABA RIVER (SOUTHEAST AFRICAN CONTINENTAL MARGIN) by E.R. Hay PLEISTOCENE PHOSPHORITES OFF THE WEST COAST OF 122 by G.F. Birch XIV GUIDE TO THE SEDIMENTOLOGICAL USAGE OF THE 132 COULTER COUNTER MODEL TAII AT THE UNIVERSITY OF CAPE TOWN by F. Camden-Smith and A.K. Martin XV CONSOLIDATED LIST OF PUBLICATIONS OF THE MARINE 147 GEOSCIENCE GROUP (i) INTRODUCTION by R.V.Dir.gie

Having been behind for several years with our "annual" reports, the current edition, which covers 1981-ir'82, will bring w: back on schedule so that the 1983 report will, hopefully, pertain only to 1983! During the period under review we bad several changes in personnel with the result that there have been, and will continue to be new- faces on the 5th floor of the Earth Sciences Building. In addition, new research projects and prospects have beer, opened up. I hope we have emerged from this unsettled phase and that 1983 will be a year of exciting expansion. The Geological Survey tear;; were most affected by loss of manpower: Dave Salmon in mid-1981 (micropalaeontology, to Anglo-American), Andre du Plessis at end of 1961 (geophysics, promotion to an Assistant Director in ), Gavin Birch in March 1982 (sedimenfology, to ESSO ) , ó.id Donne Murray in mid-1982 (geophysics, to commerce). Mike Bremner is now in charge of the GSO operation, and we hope to slowly re-build this side of our operation. In early 1981 we also lost Steve Goodlad to SOEKOR, and Keith Martin transferred to the NRIO staff, but stayed with us at UCT. Frances Camden-Smith has also left the shelter of the 5th iloor, but will continue to work on her Ph.D. thesis, which is eagerly awaited by the deep-sea researchers. To compensate for some of these losses we were joined by Simon Robson (early 1982) to work for his Ph.D., and welcomed back Dr John Rogers as SRO (early 1982). John was one of the originals when Professor Simpson started the SANCOR Marine Geology Unit in 1967, and had f:or the last 5 years been working for the G30 Bellville Office on (ii) West Coast Neogene sediments. Dave Salmon's, departure left us in bad shape with microplankton studies, but we have been very fortunate in being granted a SANCOR post-Doctoral Fellowship and acquiring the services of Dr Amos Winter from Stanford University who arrived in late 1982 on a three year contract. All this toing and froing has certainly caused disruption in our work programmes, but I think we will emerge stronger and perhaps better co-ordinated. The creation by UCT of the post of Sonnenberg Senior Research Officer in Marine Geoscience earlier this year gives us the opportunity to further strengthen our team. To summarize the current main research programmes: GSO Seismic and sediment studies on the inner continental shelf of Namaqualand (NW Cape); NRIO 1. Seismic and sediment studies on the . 2. Seismic, sediment and geochemical studies on the continental shelf along the east coast between Port Elizabeth and Mozambique; UCT 1. Seismic, sedimentological, geochemical, stratigraphic, and oceanographic studies on the contintental slope and deep- basins off SE Africa. 2. Oceanic micropalaeontological studies off SE Africa.

Overseas visits by members of our group during 1981-1982 included: Dr. B. W. Flemming; Joint Oceanographic Assembly, Halifax 1982 Dr. J. M. Bremner; Symposium on Oceanic Upwelling, Vila Moura, Portugal 1981. Prof. R. V. Dingle; Sabbatical leave to UK (4 months, Edinburgh and London University), and New Zealand (2 months sample collecting), 1981. > iii)

Scientists and support personm in the Marine Geoscience Group at the end of 1982

UCT R.V.Cingle Professor of Marine Geoscience J.Rogers Senior Research Officer A.winter SANCOR post-Doctoral Fellow E.G.Mills Senior Technical Officer S.Robson UCT Scholar Mrs F.Camden-Smith UCT Scholar Miss J.Frewin UCT Scholar Miss E.R.Hay UCT Scholar

Geological Survey J.M.Bremner Responsible Geologist R.De Decker Geologist P.Bova Geotechnician M.Woodborne Geologist

National Research Institute of Ocear.ology B.W.Flemming Senior Research Officer A.K.Martin Research Officer

Support Staff Mrs E.G.Krummeck (UCT) Secretary Mrs S.M.O.Sayers (GSO/UCT) Research Assistant Mrs S.N.Smith (GSO/UCT) Research Assistant Mrs h.Brandstatter (GSO/UCT) Research Assistant Mr H.Fortuin (NRIO) Research Assistant Mr M.Mahoney (UCT) Research Assistant Mr M.Smith (GSO/UCT) Research Assistant Mr V.Williams (GSO) Technical Assistant Mr R.O.Van Wyk (GSO) Technical Assistant (iv)

Cruises undertaken by personnel in the Marine Geoscience Group

1. UCT R/V Thoaas B. Davie 1981 days 1982 days

424, 8/3-28/3 21 430, 20/8, 23/8 2

2. GSO Cruises on Seedelwer 1

SAD 4/81 4/5-25/5 22 SAD 4/82 19/4-26/5 38 SAD 9/82 6/9-22/9 17

3. NRIO R/V Meirinq Naude

81-01 19/1-30/1 12 82-4C 12/2-22/2 11 81-08 11/5-22/5 12 82-4E 2/3-10/3 9 81-11 29/6-10/7 12 82-05 29/3-2/4 5 81-19 9/11-20/11 12 82-11 19/5-27/5 9 82-21 17/10-22/10 5

70 117 1

I. PRELIMINARY REPORT ON THE BATHYMETRY OF PART OF THE TRANSKEI BASIN by S.H. Robson

INTRODUCTION During cruise 424 of the R.V.Thomas B. Davie in March 1982 some 1405 nautical miles of bathymetric data were collected from the Transkei Basin with a 12 Khz ELAC echosounder. This equipment has a resolution of 10 metres per millimetre of paper in deep water (more than 1000 m). Navigation was by Decca Navigator with fixes taken at thirty minute intervals. As a back-up to Decca a Satellite Navigator was operated to calculate deed reckoning positions every fifteen minutes. Satellite fixes were obtained on average every seventy five minutes (with a range from eighteen to one hundred and twenty six minutes). The continental slope along the south eastern margin of South Africa is narrow (22 km off East London - 15 km off Port St Johns; Siesser, Scrutton and Simpson 1974) and steep (up to 15°). The gross morphology of the margin is the result of the north eastern edge of the Falkland Plateau sliding past the African coast as Africa and parted following strike-slip faulting approximately 127 m.y. B.P. The line along which the two continents separated is now marked by the Agulhas-Falkland Marginal Fracture Zone. This zone is defined by both a magnetic anomaly (Rabinowitz and LaBrecque 1979) and by a basement ridge which dams sediments and truncates pre-opening structures.

Results and Discussion. From the bathymetric data collected on the cruise fair charts were produced and contoured. These were then merged with the pre-existing bathymetric information to produce Figure 1. I

Whilst there is relatively more information from the area off East London, the variation in the style and complexity of the bathymetry is real. The central region in Figure 1 (the IJ.4 km long section of the continental slope between the Great Kei River and the Mtata River) is both smoother than the area either side as well as being relatively free of submarine canyons. Just to the south-west of the Keiskamma River but not shown in Figure 1 is a large bathymetric discontinuity which is to be investigated on subsequent cruises. The fundamental change in nature between the central and flanking regions is thought to reflect a number of basement features. The port on of continental margin shown in Figure 1 is positioned over a buoyant basement feature, (the Transkei Swell). The northern end of the Transkei Swell ic at the Egoso Fault which lies just to the north of Port St Johns. In the region immediately around Port St Johns there is a smell family of faults intersecting the coast and creating small outliers at Mbotyi and Mngazana (Dingle, Siesser and Newton, 1993). The area of more complex bathymetry seen off the coast of Port St Johns (Fig.l) may be linked to the presence of these faults (Dingle, pers. comm. 1982). Straddling East London are several fault controlled outliers (Kidd's Beach, Kwelera River and Kei River; Dingle, Siesser and Newton, 1983). There may be a genetic relationship between these faults and the steep sided submarine canyons seen in Figure 2. The presence of these faults may also provide the source of the earthquakes which triggered the slumps tentatively identified on tracks A and B (Fig.3).

Intensive regional scale bathymetry was carried out fn the area off the coast between th>? mouths of the Keiskamma and Great Kei rivers (Fig.l). This sector of the continental slope is dissected by at least 18 steep sided submarine canyons with 3

trends approximately perpendicular to the coastline. These canyons can be traced on the bathymetric map (Fig.l, from the 1500 m contour to the ) for distances ranging from 6 to 83 km with a mean of 41 km. Track C (Fig.2) which runs roughly parallel to the continental slope shews the relief generated by 5 of these submarine canyons. The walls of the deepest of these canyons are 880 m high and the others are 400 to 500 m high. These ridges and canyons are frequently asymmetrical in cross section although the direction of the asymmetry is variable. The steepness of their flanks appears to be remarkably consistent (inclinations range from 1 in 7 to 1 in 10) and they are distinctly steeper than the continental slope in the canyoned area which averages 1 in 31. In the uncanyoned regions, (the central sector between the Great Kei and Mtata rivers), on the lower continental slope and rise (from the 1500 m isobath down to the abyssal plain) the inclination is of the order of 1 in 60. If the lower continenta1 slope and rise in the uncanyoned area is taken from the 2500 m isobath to the abyssal plain, which lies at a depth of approximately 3700 m, then th" inclination is 1 in 140. Tracks A and B (Fig.3) which run perpendicular to the coastline in the canyoned area show that the continental slope can be divided into upper and lower sections. A definite change in the angle of the continental slope is apparent on both tracks at a depth of approximately 2500 m. The jpper slopes (here defined to be from th .; shelf break down to the first prominent cnange in the angle of the slope (1870 m on track A and 2175 m on track B) have angles of 1 in 19 and 1 in 17 respectively. The lower slopes have angles of 1 in 37 and 1 in 29. Prominent bathymetric irregularities occur on both tracks A and B (marked by arrows, Fig.3). These "notches" occur at approximately constant depths on nearly all the tracks 4

perpendicular to the coastline anu it is possible that they are features that run along the slope rather than down it. They are tentatively identified as slump structures although they may be younger subsidiary canyons cutting down and across the older canyons. In subsequent cruises an attempt will be made to obtain reflection seismic profiles across these structures. Evidence from New Zealand (Lewis 1971) and Israel (Almagor and Wiseman 1977) has shown that slumping can occur on continental slopes with inclinations as low as 1 in 60. Slumping is known to occur on the south eastern continental margin of South Africa (Dingle 1977). At depths of 600 m and 1800 m on track A and 2000 m on track B are forms that can be interpreted as tensional depressions and at a depth of 3600 m at the base of track B are additional features that tend to confirm this interpretation, (see below). The abyssal plain - continental slops contact is quite distinct on most profiles and on several (track B, Fig.3 for example) the contact is marned by a 7 km wide hummocky terrain of jumbled 30 m to 50 m metre high structures. These may represent slump debris or compressional features from the toe of the slump. In the uncanyoned area off the mouth of the Mbashe River (Fig.l) the contact is more gradual and is a 43.5 Km wide transitional ftaa from the 3500 m isobath to the edge of the abyssal plain.

Future Plans. In research cruises planned for the next few years it is intended to gather more bathymetry over the rest of the Transkei Basin, to confirm or disprove the presence of large slump structures along the continental margin between the Arch and the Port Shepstone Arch, (the natural boundaries for this section of the continental margin) and to obtain seismic reflection profiles across the Transkei Basin. 5

Additional current meters will be deployed, station 6570 - current meter site no.5, being only the first of a series of planned current meter stations intended to trace the sources and routes of suspended sediment input to the Transkei Basin.

References. Almagor, G. and Wiseman, G. 1977. Analysis of submarine slumping in the continental slope off the southern coast of Israel. Mar. Geotech 2; pp 349-388. Dingle, R.V., Siesser, W. and Newton, R. 1983. Mesozoic and Tertiary Geology of Southern Africa, Rotterdam, Balkema. (in press) . Dingle, R.V. 1977. The anatomy of a large submarine slump on a sheared continental margin (SE Africa) . Jl^ geol. Soc. Lond. 134; pp 293-310. Lewis, K.B. 1971. Slumping on a continental slope inclined at 1'- 4'. Sediment. 16; pp 97-110. Rabinowitz, P.D. and LaBrecque, J. 1979. The Mesozoic South and evolution of its continental margins. Jl Geophys. Res. 84; BH, pp 5973-6003. Siesser, W.G., Scrutton, R.A. and Simpson, E.S.W. 1974. In Burke, C.A. and Drake, C.C. (ed). The Geology of cjntinental margins. Berlin, Springer-Verlag. ui 3 33°0050 S 28° 50 60'E 33°38.80'S 27°59.00'E

—2000m

SCALE vert. exag. X50

-2400

Figure 2 16,00 KEY _ i sea level time(hr) ^deptrKm) 0 20km 17 00 \ . 1 ... 1 \ L400 scale: vert. exag. X34 Vv \ tensional * \ —goo depression? \ \l80O V M200

06,00 0500 \ 1900 --> >-sealevel \ L1600

\ 0400 \^VNv <4B TRACK A \ -2000 \ Woo A -800 \ L2400 upper slope -• \ „-- *\r\ \ 0300 V\ 2100 V M200 \^ C-2800

A V 22.00 S. -1600 N^ I-320O

0200 TRACiitAcrKv B^o^ l //A \L200 O VXC-360 2300 0 tensional depressions I/ \ ^

VA 01,00 \ L2400

\ -2800

lower slope s\ 24,00 \ L3200

\ 23.00 \>v "-3600 compressional feature seen as hummocky terrain •> V^__

'iyure 3 6

II. DISTRIBUTION OF BENTHIC OSTRACODS DURING THE EARLY

SEPARATION OF SOUTHERN AFRTCA AND THE FALKLAND PLATEAU by R.V. Dingle

INTRODUCTION This contribution summarizes some of the conclusions reached in a micropalaeontological study of mid-Cretaceous ostracod faunas obtained from DSDP sites 327 and 330 on the Falkland Plateau, dredged samples on the Agulhas Bank, and outcrop samples from Zululand. A detailed account of sampling localities, ostracod systematics, and palaeoenvironment£:l analyses has beon submitted to che South African Museum for publication in their annals (Dingle 1983). Consequently, the formal status of some taxa mentioned herein are provisional upon their publication elsewhere. In addition, previously published data from Argentina (Musacchio 1978), Madagascar (Grekoff 1963), Mozambique Ridge (Simpson et al 1974), Tanzania (Bate & Bayliss 1969, Bate 1975), India (Guha 1976), Australia (Oertli 1974, Krommelbein 1974) and southern Africa (Dingle 1969, 1971, Brenner & Oertli 1976) have been used in these syntheses. In last year's report (Dingle 1981a), I gave an account of the ostracod work that has been undertaken at UCT over the last few years, and this contribution represents continuing activity within a project concerned with Mesozoic faunas. Continental drift (as defined by the creation of between the separating continental units) probably commenced between southern Africa and South America at about 127my (Vaj.anginian) (Larson & Ladd 1973) and resulted in the formation a.id progressive enlargement of the SE Atlantic and the S Natal Valley. However, because of the large marginal offset created by the Agulhas/Falkland Fracture Zone, the two continents remained physically joined along a progressively shortening zone 7 unti' about 100my (late Albian/early Cenomanian). In a consideration of the Barremian-Cenomanian ostracod faunas of South Africa it is important, therefore, to include those of the same age from the Falkland Plateau because the Agulhas Bank, Falkland Plateau and Zululand were close together in a slowly evolving palaeogeography, and their faunas can be expected to show similarities. Figures 1 and 2a show the palaeogeographic relationships of the various West Gondwana sites mentioned above before continental separation commenced, and the purpose of this account is to briefly discuss the evolving palaeogeographies produced by early separation in relation to the ostracod faunas of the marine sedimentary basins shown in these figures. The time period with which we are mainly concerned *s Aptian to Cenomanian. Table 1 lists the species recorded from SE Africa and the Falkland Plateau, as well as their distribution in other localities within Gondwanaland.

DISCUSSION

SE Africa 26 species belonging to 22 genera (with two species unallocated) have been recorded from the Aptian to Cenomanian strata of SE Africa. The bulk of these (19 species in 15 genera) are from the Zululand area, where relatively large numbers of specimens were collected from rocks that range in age from Aptian to Cenomanian. Collections from the Outeniqua Basin consist of two samples only (TBD 1113 and 1266) which probably have a range restricted to Upper Aptian-Albian. One species only (Sondagella theloides) is common to these two areas in mid Cretaceous material, although a further two species (Majur.gaella nematis and M? hemigymnae) are common in pre-Aptian strata. o

Pclaeoecology In Zululand, a total of 21 samples yielded ostracods. Plotting smoothed population data on a Cytheracea - Cytherelloidea - Bairdiacea + Cypridacea diagram (CCBC triangular diagram), the earliest populations (Aptian IV) lie within the field predictive of water depths of 100-200 m (the 4a populations of Dingle 1981b figs.68 & 75). This plots as the deepest water population encountered in the Zululand mid-Cretaceous, and younger populations suggest progressively shallower water depths and higher energies of the sedimentary environments: Albian III and Albian IV populations lie above the 'Cytheracea line' and just within the <100 m predictive field; and the Albian VI and Cenomanian II and III populations lie within the high energy sector of the <100 m predictive field.

Biostratigraphy In the cytheracean component of the ostracod populations found in the Zululand mid-Cretaceous rocks, two groups can be distinguished: those that are known only from Zululand; and those which nave been recorded from other localities in SE Africa and Gondwanaland. The latter comprises Majungaella nematis and Sondagella theloides, which range upwards into Cenomanian III and Albian III, respectively. These are the youngest records of the two species, both of which have extensive temporal and spatial ranges outside the SE African region. The most diverse assemblages occur in the late Aptian-early Albian strata which are characterized by the presence of several apparently short- range cytheracean taxa such as Makatinella inflata, Pongolacythere striata, and two species of Pirileberis (makatiniensis and mkuzensis). It is clear from this distribution that despite the hiatus between the Makatini and Mzinene formations (Albian I is missing), there was faunal continuity in the ostracod populations across the Aptian/Albian 9

boundary. Our evidence, although slender, does however point to the establishment of an Albian-Cenomanian assemblage by Albian III times, whose most diagnostic element so far recognized is Isocythereis? ndumuensis. Makatinella tritumida, an easily recognized taxa, is the only member of the endemic cytheracean group which is known to range from Aptian IV to Cenomanian III. From these Zululand mid-Cretaceous ostracod populations, no species are known to extend into the overlying St Lucia Formation (Coniacian at outcrop, Turonian at subcrop). Of the two samples from the Agulhas Bank, only TBD 1113 contains a large ostracod fauna (92 valves) , and this is composed entirely of cytheracean types, with Arculicythere tumida (46%) and Sondagella theloides (45%), the dominant taxa. We have no other similar populations with which to make a comparison, and the known environmental preferences of the two dominant taxa appear to be at variance. Arculicythere tumida occurs in DSDP 259, 327 and 330, and in all cases was probably deposited in water depths of ca 300 m, whereas records of Sondagella theloides for which we have water depth information, have so far all suggested relatively shallow water environments of less than 100 m (Albian III in Zululand and Valanginian in the ). Their mutual presence in sample TBD 1113 may indicate that each species was near the limit of its own environmental range, and suggests a water depth of about 150 m. The only anomalous aspect of this conclusion is that both occur in relatively large numbers, which might not be expected if they were at their respective depth range limits. The fact that Isocythereis sealensis also occurs in DSDP 327 in samples that indicate deposition in water depths of ca 200 m (where it makes up 3 to 14% of the fauna) and is also present in small numbers (5%) in TBD 1113, suggests that it is Sondagella theloides that has a greater water depth tolerance than previous data indicated. 10

Regional considerations During the time period with which we are concerned (Aptian to Cenomanian), the palaeogeography in the vicinity of SE Africa underwent significant changes. Reconstructions used here are based on Dingle et al (1983), and are shown in Figure 2(a&b) (pre-breakup Valanginian, and middle Albian/Cenomanian, respectively). It is essential to consider the regional spatial and temporal distribution of the various taxa in terms of these palaeogeographies because they change from essentially inter­ continental shelf seas (pre-breakup) to marginal seas separated by deep ocean basins (Cenomanian). In addition, it must be borne in mind that there was no connection between the North and South sectors of the Atlantic Ocean across the before late Cenomanian/early Turonian times, and that the Mid-Cretaceous populations with which we are concerned can be seen as the last representatives of an Upper to Cenomanian ostracod faunal province that occupied South Gondwanaland seas (Dingle 1982, Tambareau 1982). Figure 3 shows range charts for Mid- Cretaceous ostracods that are common to more than one of the regions in Figure 2, or have close relatives elsewhere in South Gondwanaland. Dingle (1982) has given details of the whole pre-Aptian South Gondwanalcind ostracod fauna, hut here it is relevant to emphasize the distribution of two important species in terms of a pre-drift Valanginian palaeogeography (Figure 2a). In this reconstruction, which is meant to illustrate the period immediately prior to, and immediately after drifting in the Natal Valley/South Atlantic, conditions of no stdimentation, or very slow anoxic sedimentation obtained over the DSDP sites (327 & 330) on the soutnern Maurice Ewing Bank: no ostracods were recovered from the Kimmeridgian-Albian condensed sequences (or non-sequences) related to this period. However, because the Agulhas Bank, Mozambique Ridge (DSDP 249) and Madagascar 11

(Majunga Basin) are all characterized by the presence of Majungaella nematis, a shallow seaway must have bypassed the Maurice Ewing Bank on its southern and possibly northern flanks. The latter routt is suggested by the presence of marine strata of possible Valanginian/Hauterivian age on the Transkei coast at Mngazana (McLachlan et al 1976). The Zuluiand-South Mozambique area lay to the west of the coastline at this time because pre- Upper Barremian marine sediments are not known from it. Majungaella nematis also occurs in Hauterivian sediments in the Neuquen Basin of west central Argentina,(see Fig.l) which, with the presence of Sondagella theloides indicates a connection with the Agulhas Bank area. Whether this connection actually lay across the site of the present South Atlantic (for which no subsurface structural evidence has been found, but which seems the most obvious route), or via a west Pacific-SW route (there is a western exit to the Neuquen Basin) , is not known. M.nematis also occurs on the Mozambique Ridge (DSDP site 249), and in Madagascar, giving a geographical range of at least 6500km. Faunal continuity across western South Gondwanaland at the time of initial continental drifting (ca.l27my, latest Valanginian) is, therefore, an established element in the pre-Mid Cretaceous palaeogeography. The post-drift palaeogeography shown in Figure 2b is designed to illustrate the situation immediately following continental separation between the Falkland Plateau and the Agulhas Bank (latest Albian), whilst faunal details shown at the various sites cover the period mid Albian (Albian III) to early Cenomanian. By this time, the anoxic conditions over the Falkland Plateau DSDP sites and in the deep parts of the narrow South Atlantic basin had been dispelled and marine ostracod faunas had become established, at least in the former area. During the period covered by Figure 2b, final continental separation between southern Africa and the Falkland Plateau (i.e. 12

South America) took place, and after about 100my (latest Aibian) no shallow water (continental shelf depth) connections between the two areas existed in the South Atlantic. As sea floor spreading proceeded, the gap between the two areas progressively increased, presumably preventing any further contact between elements in the ostracod populations that were restricted to shallow water environments.

Mid-Cretaceous faunal links between SE Africa and the Falkland Plateau Mid-Cretaceous faunal relationships are summarized in Table 2 and Figure 2b. Localities from which data are available are different to those in pre-drift times: no marine sediments of Barremian to Lower Maastrichtian age are known from the Neuquen Basin and post-Neocomian to Santonian strata are missing from the Mozambique Ridge; whilst in Mid-Cretaceous times, marine sedimentation commenced in Zululand, and re-commenced on the Falkland Plateau. Sondagella theloides occurs in the Outeniqua Basin and in Zululand, but appears not to have spread to the Falkland Plateau when normal marine conditions became re­ established in early Aibian times, even though it originally extended farther westwards into the Neuquen Basin. Its upper limit is Aibian IV (in Zululand). The genus Majungaella, on the other hand, did migrate into the Falkland Plateau area presumably from the Outeniqua Basin which lay adjacent to it in middle Aibian times. Majungaella nematis is known to extend into the Cencmanian in Zululand, but only rare specimens of a related species are known from the Outeniqua Basin (M.cf queenslandensis) and the Falkland Plateau (M^ sp 327/16). The genus was clearly on the wane in this region because in all localities it forms only a minor component of the fauna. Sondagella and Majungaella represent residual elements of the late Jurassic-Lower Cretaceous faunas of fJW Gondwanaland, that were geographical ly very 13

extensive (see Dingle 1982) , both of which succeded in colonizing Zuiuland in the mid-Cretaceous trangression, but only the latter of which did the same on the Falkland Plateau. New elements which vigorously took advantage of the radical and evolving palaeogeographic dispensation were the genera Isocythereis and Arculicythere. Presumably because of newly established circulation patterns, and the resultant environmental disparities, the faunas of Zuiuland and the Outeniqua Basin show only weak links in these new taxa, whereas the latter and the Falkland Plateau, which were still physically joined in Albian times (although possibly bathymetrically separated by the shallow or emergent region of the Agulhas Arch/Maurice Ewing Bank) have very close links at species level. The former is represented by I.sealensis in the Outeniqua Basin and Falkland Plateau, and by I? ndumuensis in Zuiuland. These two species are not closely related, emphasizing the relative isolation of the two areas. Similarly, Arculicythere tumida occurs at the southern two localities, but is absent from Zuiuland. This species is also present in the Albian of DSDP site 259 off western Australia and indicates continuity at shallow to moderate water depths over a distance of at least 6000km. Arculicythere first appears in the Portlandian to Valanginian of Madagascar. Similar long-range connections between Australia and the Falkland Plateau are indicated by the mutual presence of Robsoniella falklandensis and Cytherura? oertlii (in the Albian of both areas). The bulk of the cytheracean ostracods which colonized Zuiuland during the late Barremian transgression have no relations in either the Outeniqua or Falkland basins, but have closer ties (albeit through Upper Jurassic and Lower Cretaceous relatives) with both Madagascar and Tanzania. One interesting characteristic of the Falkland Albian assemblages is the relative abundance and diveristy of the micro- ostracod populations. This indicates a connection to the North 14

Atlantic/European areas (e.g. Paraceratina, Eucytherura, ana Parahemicyther idea), which all the evidence points to not being via the present day South Atlantic route. It was presumably via East Africa and western Tethys. The fact that none of these distinctive taxa have been recorded from SE Africa indicates a subtle, but important environmental difference between the latter areas and the Falkland Plateau. It was certainly the most "oceanic" of the three settings that we have investigated, and may also reflect the possibly mildly anoxic waters that could have (?)periodically swept over the plateau from the still less than completely well-oxygenated South Atlantic basin. In terms of the Callovian to Cenomanian south Gondwana ostracod province to which we have previously referred, the following summary can be made: 1. In pre-Albian times, Majungaella nematis, Sondagella theloides, Amicytheridea and Progonocythere were characteristic elements, but no record of these is known from the Falkland Plateau. This is called the "old" Gondwana fauna. 2. Following the withdrawl of the sea from the Neuquen Basin, and the Albian transgression in the Falkland, Outeniqua, and Zululand areas, elements of the "old" fauna colonized Zululand and re-colonized the Outeniqua basin, but made only a limited penetration of the Falkland Plateau. New taxa appeared with this transgression (the "new" Gondwana fauna) and colonized all thiee areas, locally producing a mixed association of "old" and "new" elements. 3. This "new" Gondwana fauna was relatively short-lived (Albian to Cenomanian), because following the withdrawl of the sea from all three areas in late Cenomanian times, it was totally replaced in the ensuing Tur^.iian-Coniacian transgression by aggressive new shallow-water forms in SE Africa, and by deep-water forms in the Falklands area. Both of these assemblages have a cosmopolitan aspect, which display few characters that could be related to a 15

"Gondwana" location. An equally dramatic change can be seen in the Nt?uquen Basin when it was inundated again in the Kiddle Maastrichtian. Here, colonization progressed from the north, and its latest Cretaceous and Palaeogene faunas are closely allied to those from Brazil and equatorial West Atrica.

REFERENCES

Bate,R.H.1975- Ostracods from Callovian to Tithonian sediments of Tanzania, East Africa. Bull. Br. Mus. (Nat.Hist.) (Geology),26;163-223. and Bayliss,D.D.1969. An outline account of the Cretaceous and Tertiary foraminifera and of the Cretaceous ostracods of Tanzania. Proc. 3rd Afr. Micropal. Coll.,113-164. Cairo:National Information and Documentation Centre. Brenner,P. and Oertli,H.J.1976. Lower Cretaceous ostracodes (Valanginian to Hauterivian, from the Formation, Algoa Basin, South Africa. Bull, du Centre de Recherche de Pau, Soc. Nat, des Petr. d'Aguitaine,10;471- 533.

Dingle,R.V.1969. Marine Neocomian Ostracoda from South Africa. Trans. Roy. Soc. S_. Af r., 38; 139-163. 1971. Some Cretaceous ostracodal assemblages from the Agulhas Bank (South African Continental Margin). Trans. Roy. Soc. S^ Afr^,39;393-418. 1981a. Summary of research on South African Cretaceous ostracods undertaken in the Marine Geoscience Group at UCT, with comments on the Mesozoic biostratigraphy of southern Africa and relationships with other Gondwanide localities. Joint GSO/UCT Marine Geoscience Unit Tech. Rept.,12;86-106. 1981b. The Campanian and Maastrichtian Ostracoda of South East Africa. Ann. S. Afr. Mus.,85;3-181. 1982. Some aspects of Cretaceous ostracod biostratigraphy of South Africa and relationships with ot.ier Gondwanide localities. Cret. Res., 3; 1983. Mid-Cretaceous ostracoda from southern Africa and the Falkland Plateau. Submitted to Ann. S. Afr. Mus. , Siesser,W.G. and Newton,A.R. 1983. Mesozoic and Cenozoic Geology of Southern Africa. Rotterdam, Balkema (in press.). 16

Grekoff,N.1963. Contributin a l'etude des ostracodes du Mesozoique moyen (Bathonien-Valanginien) du Bassin de Majunga, Madagascar. Rev, d 1'Inst, franc, du Petr. et Ann, des comb, liq.,18;1709-1762. Guha,D.K.1976. On some Mesozoic Ostracoda from subcrops of Banni, Rann of Kutch, India. Proc. VI India Coll. Micropal., Strat.;84-90. Larson,R.L. and Ladd,J.W.1973. Evidence for the opening of the South Atlantic in the early Cretaceous. Nature,246;209-212. McLachlan,I.R., McMillan,I.K. and Brenner,P.W.1976. Micropalaeontological study of the Cretaceous beds at Mbotyi and Mngazana, Transkei, South Africa. Trans. Geol. Soc. S. Afr.,79;321-340. Musacchio,E.A.1978. Ostracodos del Cretacico inferior en el Grupo Mendoza, Cuenca del Neuquen, Argentina. Proc. VII Geol. Cong., Argentina, Neuquen II;459-473. Oertli,H.J. 1974. Lower Cretaceous a.,d Jurassic ostracods from DSDP Leg 27 - a preliminary account. Init. Rept. Deep Sea Drill. Proj.,27;947-965. Washington,D.C.:U.S.Govt. Print. on~. Simpson,E.S.W., Schlich,R., Gieskes,J., Girdley,W.A., LeClaire,L., Marshall ,B.V., Muller,C, Sigal,J., Vallier,T.L., White,S.M., and Zobel,B.1974. Init. Rept. Deep Sea Drill. Proj, 25. Washington,D.C. :U.S. Govt. Print. OtT~. Tambareau,Y.1982. Les ostracodes et l'histoire geologique de l'Atlantique sud au Cretace. Bull. Cent. Rech. Expl .-Prod. Elf-Aquitaine,6;l-37. Table 1.Geographical distribution of mid-Cretaceous (Aptian-Cenomanian) osttacoda from SE Africa and adjacent areas. (Pre-mid-Cretaceous occurrences are-starred). Arg. F.Plat. A.B. Zulul. M.R. Mad. NW Aus. Cytherella bensoni x C.spp x Cytherelloidea agulhasensis x C. makatirpensis x C. ndumuensis x Bairdoppilata sp x B. sp X Paracypris sp x Robsoniella falklandensis x Sondagella theloides .x* x x x* Majungalella cf queenslandensi s x M. nematis x* ?x*....x x*...x M? hemigymne x M? sp 327/16 x Arculicythere tumida x x Isocythereis sea lens is x x I? ndumuensis x Cythereis agulhasensis x Pirileberis makatiniensis x P. mkuzensis x Makatinella inflata x M. tritumida .x Pongolacythere striata x Procytherura cf aerodynamica x P. batei x P. cf dinglei x Sphaerolebris? sp.A x Monoceratina? sp x Pariceratina liebaui x Pedicythere falklandensis x Eucytherura rugosa x E. stellifera x Hemingwayella { Parahemingwayella) barker i x H. (P.) dalzieli x H. (P.) reticulata x Hemiparacytheridea ewingensis , ,x H. challenger! x Collisarboris? stanleyensis x Asciocythere? dubia x Cytheropteron bispinosa x C sp 327/18 x C? sp x Rhadinocythere? sp 327/18 x x R? striosulcata x Cytherura? oertl: x x ndet sp 1 x . sp 2 x . sp 330/1 x . sp 327/16A x . sp 327/18 x . sp 327/16B x totals 2 28 8 19 2 1 4 51 spp, 26 genera Arg. = Argentina;F.PI at. = Falkland Plateau;A.B.-Agulhas Bank ,«Zulu! . -Zululand ;M.R.-Mozambique Ridge;Mad.=Madagascor;NW Aus. = NW Austraii i . Table 2. Summary of faunal links a) SE Africa a"nd~ tïïe FalkTand Plateau

Outeniqua Basin (Ag.3k + Algoa)=OB, Zululand=Z, Falkland Plateau=FP, Neuquen=N, Australia=Au, Madagascar=M, Tanzania=T primary links * Sondagella theloides OB-Z-N* MajungaeTTa nematis. OB*-Z-N*-M* Arculicythere tumida Ob-TP-Au Isocythereis sealensi OB-tF I. ndumuensis Z Robsoniella falklandensis FP-Au PirilebeTTs spp Z-M secondary links Rhadinocythere spp FP with U.Jur.Au,T Procytherura a'erodynamica Z " Kimm. T LL dinglei..TT7 FP " Haut/Val. OB P. batei...FP similar to P.maculata from Haut.OB FP similar to Paijenborchellina spl (Damotte) Cytherura oertlii FP " Alb. Au * pre-Albian occurrences b) between Zululand» Madagascar» Tanzania

Pirileberis Z-T-M Pongolacythere Z similar tc T FIGURE EXPLANATIONS

Figure 1. Sketch of pre-drift SW Gondwanaland reconstruction (after Dingle et al 1983). Abbreviations: N=Neuquen Basin in Argentina; AB=Agulhas Bank with Outeniqua Basin off SE Africa; Z=Zululand-S Mozambique Basin; FP=Falkland Plateau; MEB=Maurice Ewing Bank; MR=Mozambique Ridge; WA=West Antarctica microplates; EA=East Antarctica. *=DSDP sites 249(MR), 227 & 230(MEB). Dashed lines are lines of eventual continental breakup. Figure 2. Palaeogeographical reconstructions of SE Africa and the Falkland Plateau: (a)pre-drift, with Valanginian to Hauterivian faunal relationships; (b)early Cenomanian, with middle Albian to early Cenomanian faunal relationships. l=shallow basin over continental crust, 2=deep marine basin over oceanic crust, 3=structurally controlled basin edges, 4=subsequent lines of continental separation, 5=continental edges, 6=samples sites. Mad=Madagascar; M=Majunguella nematis (M. sp. on the Falkland Plateau); S=Sondagella theloides; A=Arculicythere tumida; Is=Isocythereis sealensis; In = Isocythere is? ndumuensis; 1266 & 1113 are dredge stations on the Agulhas Bank, 249, 327 & 330 are DSDP sites. Figure 3. Temporal and spatial distribution of selected mid- Cretaceous ostracod in South Gondwana. Dashed lines are total time ranges from different localities; solid lines are total age ranges at the same locality. Numbers on RHS are informal species numbers. Abbreviations: AB=Agulhas Bank; Arg=Argentina; MR=Mozambique Ridge; Germ=Germany. 1 AB 'I - -*tlí^ 1 *" \ S \ \ \' s t>*

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Figure 3 17

III. PRELIMINARY REPORT ON GEOCHEMICAL STUDIES OF TUGELA RIVER MUDS ON THE DURBAN CONTINENTAL SHELF by T. Felhaber

The first two months of 1982 were spent investigating the possibility of commencing a geochemical study of the mud belts on the continental shelf off the Tugela River mouth (Natal). The mud belts had previously been defined by extensive seismic data taken in the area by the Marine Geoscience Division of the National Research Institute of Oceanology (N.R.I.O.), and reconnaissance studies performed on samples from the area showed that distinct geochemical and mineralogical differences exist between muds from two apparently separated depositional areas. The primary purpose of the study is to investigate the difference in geochemistry between the two areas in relationship to the geochemistry of the material being carried down the Tugela River. Field work consisting of sampling the lower Tugela River, its estuary, and the adjacent continental shelf was done from 24 March to 2 April, 1982. The offshore sampling was done from the R.V. Meiring Naude, the time for which was kindly arranged by N.R.I.O. Nine samples were taken from the river; 110 Shipek grab samples and 15 hydroplastic gravity cores were taken from the shelf. Following the field work, all the river and grab samples were dialysed, dried, and crushed. Powder briquettes of two types have been made for all samples for trace element analyses (totalling 220 briquettes), and fusion discs for major element analyses have been made for most of the samples (160 discs to date). Analyses by X-ray fluorescence spectrometry are currently being performed on the samples as prepared above, with many analyses already completed. X-ray diffraction traces to 18 determine t.ie mineralogy of the whole sediment samples have been run on all river and grab samples (111). Particle size analyses have been started on these samples and should be completed by November 1982. To complete the analytical work on the samples, it is still necessary to carry out analyses for carbonate and cganic matter (C-H-N), as well as X-ray diffraction for clay mineralogy on a selected subset of samples. These analyses, together with those still to be done by X-ray fluorescence, will be finished within the first few months of 1983. It has yet to be determined whether or not the cores collected will need to be analysed, since this may not be necessary for M.Sc. purposes. From preliminary interpretation carried out on the completed XRF analyses, it can be seen that well-defined geochemical trends are present in the muds. For example, concentrations of heavy metals are much higher in the nearshore region of the mud belt than out near the shelf break. Mineralogically, the outer part of the mud contains significantly higher concentrations of than the nearshore mud. These facts will hopefully be correlated with already existing data for other parameters such as shelf topography and current dynamics, as well as with the particle size distribution, to give a more complete picture of sedimentological processes and related geochemistry on the continental shelf. Such relationships may only be more definitively stated once the analytical phase of the project is completed, when statistical and geochemical interpretation can be carried out for all samples. Once this is done, the project will provide a thorough baseline study consisting of information useful to others hoping to carry out such work in other local ities. 19

IV. USING THE SCANNING ELECTRON MICROSCOPE AND ENERGY DISPERSIVE X-RAY SPECTROMETER TO DO MINERAL IDENTIFICATION AND COMPOSITIONAL POINT COUNTING ON UNCONSOLIDATED MARINE SEDIMENTS By S H Robson

ABSTRACT This paper describes a rapid and accurate method of point- counting sands and silt-size in unconsolidated open-ocean sediments. As traditional techniques for this operation cannot be employed on the fine-grained material which frequently forms the bulk of deep sea marine sediments, an alternative method has been sought. The method described makes use of equipment known as QUANTEX-RAY comprising a computerised data acquisition and reduction system designed for use in X-ray energy spectrometry and used in conjunction with a scanning electron microscope (SEM) . Grains that cannot be identified by their visual morphology in the scanning electron microscope are analysed by X-ray spectrometry. Spectra are acquired in 200 seconds or less and processed by a sequence of software routines under semi-automatic control producing a listing of oxide concentrations as the final result. Each user must customize the control programme and operating conditions to suit his requirements.

INTRODUCTION The technique described is intended to allow rapid and reliable identification of grains commonly found in marine sediments. It is not a method for carrying out rigorous quantitative mineralogical analyses. Given the variability in oxide concentrations found in many minerals (e.g. plagioclase feldspar

Si02 68-43 wt%, A1203 37-19 wt%)(Deer, Howie and Zussman, 1966), 20

the accuracy of the method is acceptable. Unconsolidated marine sediments from sand to silt (2000-4um) are mounted on standard aluminium scanning electron microscope (SEM) stubs topped with carbon discs. If the sample is in suspension then an easy mounting method is to syringe filter the suspension through a 45 micrometre pore size cellulose membrane filter (obtainable in 13 mm diameters that fit exactly onto standard SEM stubs). The filter paper with its randomly distributed grains, is then mounted directly on the carbon disc on the top of the stub with a drop of organic base glue. After carbon coating, the sample is ready for analysis. It should be borne in mind that the material on the SEM stub can be simultaneously examined for other sedimentological purposes such as the identification and classification of microfossils and the surface textures of mineral grains. Although the stub has been carbon coated for analysis this does not detract from the quality of photographs taken on the SEM. The spectrum acquisition and processing procedure can be run by a .simple 10-step control program that ensures that each analysis is performed under the same operating conditions in the minimum possible time, whilst permitting the operator to continue point counting further grains once the spectrum has been acquired. The control program is called "AT0,G0" and can easily be modified to suit the individual user. The steps controlled by AT0,G0 are listed and described in the discussion.

DESCRIPTION OF THE TECHNIQUE Carrying out a mineral identification and compositional point count routine may require four separate operations. These are (a) selecting grains and (b) identifying them, if the grain cannot be identified by shape, texture, cleavage etc., then it is necessary to (c) acquire a spectrum of thp grain and (d) to analyse and 21

interpret the spect'um. (a) Grain selection. With the SEM at a set magnification the stub is traversed along a line chosen at random and all grains encountered in the field of view are identified and counted until the required number has been examined (^ay 300). This method is called the Fleet method of point counting (Fleet, 1926) and results in a number percentage of the various components. The probable error in percent individual components at the 2-sigma

-1 1 2 confidence level will be Eg5>4=2(p(100-p)n ) / , where E is the probable error in percent, n is the total number of grains counted and p is the percentage of n of an individual component (Galehouse, 1971). For the recommended 300 counts per sample the worst case error will be less than 6% for components representing between 25% and 75% of the sample by volume. The reliability improves for components forming less than 25% or greater than 75% of the sample. It is therefore recommended that a large number of categories (say 8-12) be set up so as to reduce the proportion of the sample represented by any one component (Van der Plas and Tobi, 1965). (b) Grain Identification. With experience, a large variety of open ocean sedimentary components can be identified by shape and texture alone. Unusual looking grains can be photographed and analysed so that similar grains can be identified later. Most biogenic material is more easily identified by SEM than by light microscope. Unfamiliar types can be analysed and photographed in the same way as mineral grains. The composition of unidentifiable grains or those too small to be recognised visually is determined as follows. Centre and magnify the grain until it nearly fills the field of view. Select a representative surface to analyse (one free of secondary growths etc.), switch tl '•'!* to reduced raster and adjust the stage until the area to be analysed completely fills 22

the field of view. Acquire the spectrum recording counts for as long as is necessary to produce a spectrum that can be interpreted within the confidence limits you require. In practice this need rarely exceed 200 seconds and in many cases it can be identified in less time. If the spectrum appears to be particularly complex then the acquisition time may be prolonged to increase the analytical confidence. Whilst spectrum acquisition is in progress there are several optional hardware element identification procedures that may be manually performed as a check on the software. (c) Spectral Analysis. Analysing a spectrum consists of two operations; ascertaining which elements are present, and determining their relative concentrations. The identification of elements (IDE) subroutine runs automatically once initiated. The spectrum is scanned, peaks are identified and marked and a print out is automatically produced listing all x-ray lines present by chemical symbol as a function of x-ray energy. Marked x-ray lines in the spectra are matched with x-ray lines possible at those locations. IDE automatically marks all peaks located in the spectrum which contain a specified amount of data. A peak is detected and diagnosed as a peak above background when the peak is 1.3 times background. This value is set by the user with the set sensitivity command ("SETSE") and is a compromise between classifying background noise as a peak and missing out real peaks of low intensity. Even at a sensitivity of 1.3 IDE will occasionally interpret noise as a peak and not all peaks should be accepted as such without question.

Having located all peaks of interest in the spectrum IDE matches x-ray lines to marked locations according to a list of possible x-ray lines stored in memory. A "match" is obtained when an x-ray line is within a certain range of a marked peak. This range is determined by the set width command ("SETWI"). Matching 23 regions are defined according to the following equation +/- ev=WI*EV/2 where +/- ev is the number of electron volts to right or left of the marked channel that defines the matching range, WI is the window value set by the width command (recommended value is 2), EV is the spectrometer range setting which is similaily set by *-he user, (a value of 20 is generally used for geological work). In this application the equation will match peaks within l'b electron volts of the marked channel. As each peak is located and accepted, a single channel at the location is marked in yellow on the video screen so that the user can visually confirm that all peaks of interest have been identified. The list of "accepted" peaks is then scanned and illogical matches are eliminated, if say Cu K-beta is an accepted possibility for a spectral peak then the Cu K-alpha line must also be present in the spectrum given the following conditions: that the operating voltage is high enough to excite the Cu K- alpha transition, that its energy is within the energy range of the spectrum ar>^ thzc the counting period was of sufficient duration. If Cu K-alpha is not found then Cu K-beta will be rejected as an illogical match. Once all "illogicals" have been eliminated the list of possibilities is printed out along with approximate net intensities, (peak counts minus background counts) for each peak. In some cases the software cannot resolve peaks or confirm that one element was present and not another, in which case both possibilities will be listed and the user must use his own informed judgement or improve the operating conditions in order to make the final determination. Where one is working on samples of known origin it is usually possible to exclude certain elements from consideration (eg. inert gases, lanthanides etc) by entering them into memory under the set no element command ("SETNOEL"). This will reduce the number of unresolved element identifications to a minimum and 24

accelerate the analytical procedure. The IDE routine stores an element list from the list of accepted "logical" matches and once the user has accepted or modified this list the computer is ready to proceed to the final step in the analysis, determining the concentrations of the elements identified in the spectrum, (d) Calculating Element Concentrations. The sub-routine Approximate, ("APP") performs a rapid semi-quantitative analysis by calculating theoretical standard intensities for each element of interest from parameters stored im memory and an electron column quantitation programme which converts net peak intensities to concentrations. The programme calculates theoretical standard intensities for each element and then compares them to the element peak intensities in the sample spectrum. It then approximates the elemental concentrations. In carrying out the APP routine, escape peaks and background in the spectrum need to be removed. Escape peaks in a spectrum are generated by lost x-ray energies originating from the excitation of silicon atoms in the detector. They always lie 1.74 Kev down the energy scale from the parent x-ray and are a function of the energy and intensity of the parent peak and of the geometry of the detector. The QUANTEX-RAY command ESC identifies and returns the escape peak to its parent peak. Background in the spectrum may be defined as counts due to unwanted x-rays or electrons entering the detector and is mainly due to the continuum of x-ray energies produced by deceleration of the primary beam in the specimen. Additional sources of background are sum peaks, incomplete charge collection, system peaks and stray electrons. In order to calculate peak to background intensity ratios correctly it i J neccessary to predict the shape of the background. Under the APP command a Background Filter (BKF) subroutine is carried out. This automatically models the 25

background ard extracts the peaks by a weighting process sensitive to the rate of slope change within the spectrum. The filter passes over the spectrum on a channel by channel basis filtering areas of low variation in slope (i.e. background areas) out of the spectrum. For rigorous analyses there are superior but longer methods available for removing the background from the spectrum. The final step of the APP routine is an automatic print out which lists the following: (1) chemical symbol and x-ray line of each element analysed, (2) weight per cent (concentration) calculated for each element with a total at the bottom of the column, (3) atomic per cent for each element, (4) precision, the probability that the measurement lies within levels of confidence represented by sigma (or standard deviation, set by the user with the set sensitivity command), (5) the K-ratio, the intensity of the peak divided by the intensity of the theoretical standard peak, (6) the number of iterations, this is the number of times the APP routine had to repeat the fundamental-parameters calculation to correct the calculated weight percent, the lower this value the better the analysis (KEVEX CORPORATION, 1980). As we are dealing with minerals which are nearly all oxides, the APP routine can be instructed via the set oxide command ("SETOX"), to process the analytical data in terms of oxides. This causes the APP print out to list oxide formulas (predetermined by the user with the set formula ("SETFO") command), and the oxide concentrations as a percentage with a total at the bottom of the column. Using this printout and a list of likely deep sea sedimentary minerals and their compositions, unknown grains can be identified.

DISCUSSION. For analysis of unconsolidated deep-S9a sediments the 26 elements listed below permit identification of all commortly occurring marine minerals {Table 1). Element List: Si,A1,Fe,Mg,Ca,Mn,Ti,Na,K,P,Cr,Ta,Zn,Zr,Sn,Th,

Table 1» Minerals that can be identified from the above list of elements. quartz K-feldspars pyroxenes amphiboles zeolites monazite micas garnets tourmaline sillimanite andalusite kyanite epidotes olivine sphene zircon staurolite chlorite spinels ilmenite tutile pyrophyllite goethite gibbsite carbonates apatites cassiterite anatase

Some of the minerals listed above contain elements not appearing in the element list. These additional elements occur only as minor components however and it is generally possible to identify the mineral from its major constituents alone. The control programme (ATO,GO) described below inludes a step which recalls the raw spectrum after completion of the analysis. Should the analysis appear inadequate or its interpretation difficult the user may then either store and/or re-analyse the spectrum. The system offers a range of alternative spectral analysis techniques that should permit the identification of most common minerals. The automatic control programme (ATO,GO) consists of the following 10 steps: 1 CLR: Clears old data from the video screen and temporary memory. 2 SETEL: Calls up the \. 3t element list stored. 3 ENTER: Effectively deletes element list. 4 ACQ1: Acquires spectrum. 27

5 WAI: Programme pauses whilst the spectrum is acquired, the programme will not continue until either a preset counting time has been achieved or the operator instructs the acquisition to cease by means of STOP command. 6 ESC: Removes escape peaks from the spectrum. 7 REA: Stores a copy of the spectrum in temporary memory. 8 IDE: Identifies elements present in the spectrum and creates a new element list. 9 APP: Using the element list created by IDE, calculates concentrations of the elements present in the sample and prints out the result on a line printer. 10 RST: Retrieves the original spectrum from temporary memory so as to allow the operator to either store the spectrum on disc or re-analyse it.

ACKNOWLEDGEMENTS. The manuscript was reviewed by Dave Crawford, Richard Dingle, Rowena Hay and John Rogers. Dave Crawford and Dane Gerneke of the Electron Microscope Unit provided all the training and a lot of advice. Discussions on the technique were held with Frances Camden-Smith and on sedimentary petrology with John Rogers.

REFERENCES.

Deer,W.A., Howie,R.A. and Zussman,J. 1966. An introduction to the rock-forming minerals. London. LorTgman. Fleet,W.F. 1926. Petrological notes on the Old Red of the West Midlands. Geol. Mag. 63:505-516. Galehouse, J.S. 1971. In: Carver,R.E. ed. Procedures in Sedimentary Petrology. New York: Wiley - Interscience. KEVEX CORPORATION. 1980. QUANTEX-RAY Instruction Manual. 2nd ed. Van der Plas, L. and Tobi, A.C. 1965. A chart for judging the reliability of Point Counting Results. Amer. Jl. Science, 263: 87-9P. 28

VI. BASEMENT MORPHOLOGY AND UNCONSOLIDATED SEDIMENT IN ALGOA BAY by J.M.Bremner and A.du Plessis

ABSTRACT Two clearly-defined, pre-Holoce.ie drainage channels incise the basement of Algoa Bay, one extending from the mouth of the Sundays River and the other, from the region around the and Rivers. In addition to these features, a number of prominent ridges and depressions occur, some of which derive from wave erosion at lower sea-level times, and others being a reflection of basement structures. Fluvial bedload introduced into Algoa Bay since the start of the Flandrian transgression is estimated to be 23 800 X 10^ metric tons whereas unconsolidated sediment in the coastal dunes, and within six offshore deposits totals 53 517 X 10*5 metric tons. It is therefore deduced that at least half the sediment in the bay is relict.

INTRODUCTION This report constitutes the fifth in a series of related articles on Algoa Bay. All these reports, together with additional information, will eventually be synthesized into a single Geological Survey Bulletin. The reports already completed are: Bremner,J.M. 1978. Sisrficial sediments in Algoa Bay. Tech. Rep, jt geol. Surv./Univ. Cape Town 10: 6-74. Bremner,J.M. 1979a. Sediment dispersal processes in Algoa Bay. 4th Nat. Ocean. Symp. Cape Town 10-13 July, lp. Bremner,J.M. 1979b. The planimetric shape of Algoa Bay. 29

Tech. Rep, jt geol. Surv./Univ. Cape Town 11:107-117. Bremner,J.M. 1979c. The bathymetry of Algoa Bay. Rep, geol. Surv. 1980-0007: 1-13 (Open File). The purpose of this article is to present a number of mfips that were assembled from shallow-seismic work undertaken in Algoa Bay. Included in the report are our first preliminary interpretations on the disposition and abundance of unconsolidated sediment in the bay and the surrounding environs. At a later stage in the programme, the nature and structure of underlying bedrock material in the bay will be investigated as well.

METHODS Eight days of continuous-seismic reflection-profiling were undertaken in Algoa Bay in September 1976. The survey consisted of 57 profiles which were chosen to coincide with DECCA lines oriented most closely in a direction at right angles to shore. The lines were spaced approximately 1-^/2 km apart and covered the area enclosed between longitudes 25° 34'E (near Cape Recife) and 26° 30'E (near Cape Padrone); and from a depth of about 10 metres co latitude 34° 06'S (Fig.l.). The total length of the seismic traverses amounted to 1620 km. Instrumentation included an 82 cm3 Bolt airgun and a hydrophone array, which were trailed at opposite sides of the ship's stern. Returning acoustic signals were first filtered and amplified before being plotted on an analogue recorder as 2-way travel time. In places, up to 60 metres penetration was obtained but resolution was never better than 2, and often no better than 5 metres.

DEPTH TO BASEMENT A contour map showing the depth of basement below sea level 30 is presented in Figure 2. The term 'basement*, as used in this report, is meant to include all consolidated materials of pre- Holocene age. It has been shown (Bremner, 1979c) that excellent correlation exists between sea floor depths obtained wth a 3,5 Khz seismic profiler, and soundings obtained by the . However, to avoid the minor progressive, constant and irregular differences that did occur, SAN data were used in preference to the seismic seafloor-depths in establishing depth to basement. This was done in the following way: A transparent overlay of the seismic traverses in Algoa Bay was superimposed over a bathymetric map (compiled from SAN data) of the same scale. On the right-hand side of each time-mark (15 minutes=approx. 2 km) on the ..eismic lines was scribed the unconsolidated sediment thickness (measured from the records). The depth-to-basement values were calculated by adding sediment thickness values to the bathymetric depths and these values were then scribed on the left-hand side of the time-marks. These values are considered accurate to within 1 metre.

The most prominent topographic features in the basement are now briefly described.

Pre-Holocene drainage channels Along the eastern flank of Riy Bank and heading northwards toward the mouth of the Sundays River is a clearly-defined elongate depression (Fig.2). In places this channel-like feature is wiae and shallow but elsewhere it appears to be deeply incised in the basement. Similarly, on the western flank of Riy Bank is another channel, but this time its morphology is less distinct. It does, however, seem to bear northwestwards toward the Coega River with a number of side branches coming in from the east, and a shallow depression extending westwards toward the mouth of the 31

Swartkops River. Besides these two clearly defined river channels in the bay, there is some evidence to suggest the presence of a third one further to the east. M. Marker of Fort Hare University has discovered from onshore bore-hole data, that a broad basement- depression extends northwards beneath the dunes from a point roughly mid-way between the Sundays River and Woody Cape (pers.comm., 1979). She is of the opinion that this channel may have been eroded by the Boesmans River before it became deflected to its present course. Note that the mouth of the Boesmans River is presently located approximately 12 km east of Cape Padrone. Offshore, there are a number of isobath inflections in the basement which would, in fact, link up with Marker's onshore basement depression. However, the almost parallel orientation of survey lines in this area (Fig.l) with the surmised offshore channel has made accurate definition of the latter extremely difficult. One is therefore limited to saying that a continuous sub-surface channel from the Boesmans River to the middle of Algoa Bay seems to be a likely possibility. The drainage system developed in Algoa Bay basement was cut at a time of low sea level, presumably during the Pleistocene. Maximum sea level withdrawal can be traced with certainty along the Sundays River channel to a depth of -105 metres. As sea level rose during the Flandrian, the valleys became choked with imported sediment and excess material was transported by littoral currents and wind to areas of entrapment. Channels clearly identified on the seismic records are shown pictorially in Figure 3.

Basement ridges and depressions Besides the channel-like depressions just described, several other morphologic features occur in the basement. The most prominent of these are two 32 ridges oriented at right angles to each other at Riy Bank. One of the ridges strikes north-northwesterly from the bank towards the island triad of St. Croix-Brenton-Jahleel, and the other strikes in an east-northeasterly direction toward Bird Island (Fig.2). Whereas the former ridge is broad and separates the Sundays and Coega drainage channels, the latter is narrow, well- defined at 70 metres depth, and cut by the two drainage channels. The most important basement depressions are a broad bathymetric embayment between the Sundays River and Bird Island, and a narrow, linear valley at 125 metres depth on the south- southwest side of Bird Island. The former has served as a receptacle for the accumulation of Recent sandy sediment and was formed by coastal erosion from larger-than-normal waves being allowed to impinge along this section of the shoreline (Bremner, 1979b). The latter, at 125 metres depth, is the locus of Recent muddy sediment accumulation (Bremner, 1978), whereas the nearby more elevated areas with fairly high concentrations of glauconite, support very little unconsolidated sediment at all. The broad inter-riverine ridge and adjacent broad depression were probably formed at a time of high sea level; the ridge being protected from wave action by the Riy Bank massif, and the depression being exposed to south-westerly swell. At lower sea level times, e.g. at various periods during the Pleistocene, subaerial weathering and erosion would have accentuated these predetermined landforms. On the other hand, the narrow ridge at -70 metres and the narrow valley at -125 metres are parallel features, and probably reflect a structural phenomenon in the basement fabric.

UNCONSOLIDATED SEDIMENTS Large deposits of sediment exist in the coastal embayment of Algoa Bay predominantly infilling depressions in the solid 33 basement, and occasionally lying in the lee of headlands and islands. The onshore deposit consists essentially of a continuous, elongate dune-belt adjacent to the strandline, and extends for 75 kms from the Swartkops River to Cape Padrone. Material exchange between the two environments is no doubt quite extensive, however, they are discussed separately here in order to facilitate their description.

Offshore Except in the vicinity of Cape Recife, Riy Bank and Bird Island, the sea floor of Algoa Bay is variably blanketed by an extensive layer of unconsolidated sediment. In places, the sediment veneer is thin, probably less than */2 metre e.g. to the south of Riy Bank and Bird Island, whereas in other areas, thicknesses in excess of 30 metres exist e.g. in the Sundays River channel north east of Riy Bank (Fig.4). The texture and composition of these sub-bottom sediments are not known but they probably have similar properties to those at the surface (Bremner, 1978). Two maps have been compiled to show unconsolidated sediment thicknesses in Algoa Bay. The one includes material infilling river channels in the bay (Fig.5), and the other excludes it (Fig.6). Clearly evident in the former diagram is Sundays River material flanking the broad ridge between Riy Bank and the island triad. However, not so clear is material infilling the Coega River channel because here, at least two thick sedimentary deposits (Fig.6) tend to mask the channel material. In all, six discrete bodies of sediment are recognized in Algoa Bay (Fig.7). Their modes of accumulation are envisaged as follows: Deposit I - Muddy sediment transported northeastwards by bottom currents from the -125 metre trough, and sandy sediment 34

carried eastwards past Woody Cape by littoral drift. Deposit II - Sandy sediment transported eastwards by littoral drift from the mouth of the Sundays River at a time when sea level was approximately -90 metres. Negligible material was transported across the narrow basement ridge described earlier at -70 metres. Deposit III - Sandy material derived from the Sundays River and carried northeastwards by wind and littoral drift as sea level rose to its present position. The broad basement concavity described earlier between the Sundays River and Bird Island served as an ideal receptacle for the entrapment of this unconsolidated sediment. Deposit IV - Primarily material infilling the 'Sundays River channel'. The bottom portions of the channel probably contain boulders and other coarse sediment whereas the surface material is sand-size. Deposit V and VI - Sediment derived from the Swartkops and Coega Rivers from the time sea level was at approximately -90 metres until it reached its present position. The two deposits correspond to the Sundays River-derived deposits II, III and IV. Deposit V, being in deeper water, is older than deposit VI. Very little material from these deposits escaped across the broad ridge separating the Coega and Sundays River channels because of protection afforded by the Cape Recife headland. The areal coverage, volume and mass of individual deposits is given in Table 1. Areas were calculated by counting millimetre squares and multiplying these values by a factor derived from the scale of the map (1:150 000) to give square metres. Volumes in cubic metres were then calculated by multiplying the individual areas by the midpoint of each thickness zone i.e. 2,5 for 0 to 5 metres; 6,5 for 5 to 8 metres; 35

9,5 for 8 to 11 metres; 12,5 for 11 to 14 metres; and 16,5 for 14 metres upwards. The mass of the deposits, in metric toii.^. «'as obtained by multiplying individual volunes by the average density of the sediment (2,68 gm/cm3).

Onshore An extensive onshore dune system, averaging 2 km in width and approximately 75 km in length, extends along a major portion of the Algoa Bay coastline (Fig.7). The dune area was measured by counting squarr millimetres, as with the offshore deposits, and is reported together with its volume and mass in Table 1. In order to obtain a realistic value for the volume of the dune belt, its lengul» was divided into 15 equal parts, and the thickness in each measured from 1:50 000 topo sheets. This was done by subtracting the lowest reported elevation from the highest (both usually being found on the landward side of the dune belt), and dividing this value by two. In this way, minimum, maximum and average thicknesses were calculated to be 3,7 metres, 46,3 metres and 26,1 metres respectively. The mass of each part was found by multiplying its volume with the average sediment density (2,68 gm/cm3) .

Fluvial input The catchment areas, effective annual sediment yields and effective annual bedload yields of the two main river systems entering Algoa Bay, are reported in Table 2. Calculations were based on data published in Midgley and Pitman (1969) as sollows: The drainage areas of the Swartkops and Sundays Rivers are given in Regions M and N respectively (Midgley and Pitman, 1969, p.100). Sediment yields were derived using the High-4 curve (Midgley and Pitman, 1969, Fig.6,4, p.40) employing the following conversion factors: 1 morgen foot = 2610,699 m^ and 1 m3 = 2,68 36 metric tonnes sediment. A conservative estimate of the bedload yield was used viz. 5% of the total yield.

CONCLUSIONS Assuming constant environmental conditions since the start of the Flandrian transgression, bedload introduced into Algoa Bay by the Sundays and Swartkops Rivers in Holocene times amounts to 17 000 X 1,4 X 106 = 2/S 800 X _106 metric tons. Unconsolidated sediment in and around Algoa Bay, i.e. both offshore and onshore, amounts to 53 517 X 10^ metric tons. In other words, there is 2*/4 times as much sediment in the Algoa Bay environs as has been introduced during Holocene times from fluvial sources. This implies, within the accuracy limits of the calculation, that at least half the sediment, particularly in the offshore areas of Algoa Bay, is relict. A suggestion of calcrete/silcrete presence has been observed on seismic records from the broad ridge containing the nearshore islands St. Croix, Brenton and Jahleel. Similarly, 'hard' sediment is reported on South African faircharts in areas deeper than about 100 metres. The conclusions drawn take no account of sediment recycling between various deposits and the sediment sources. The figures quoted are therefore only considered to be a first approximation.

REFERENCES

Midgley,D.C. and Pitman,W.V.1969. Surface water resources of South Africa. Rep, hydrol. res. Unit Univ. 2/69:1-54. O)

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VII. GEOPHYSICAL INVESTIGATIONS IN THE VICINITY OF PORT ELIZABETH HARBOUR by

J.M. Bremner

ABSTRACT

An area of aproximately 30 km2 off P.E. harbour has been surveyed with side-scan sonar and seismic-reflection profiling. The sonar imagery shows most of the sea floor to be weakly- reflective, smooth sediment, and samples indicate that it consists of fine sand of terrigenous/calcareous composition. Fluvially-eroded basement (probably sediment) underlies two Quaternary horizons called Units 2 and 1. Unit 2 is aeolian in origin and dates from the Wurm Interglacial. It is best preserved in basement valleys in the northern part of the study area, and in the lee of a rocky bank located approximately 4 km southeast of P.E. harbour. Unit 2 became vegetated during the Wiirm II pleniglacial and was subsequently eroded during the Flandrian transgression. The locus of deposition of this material, Unit 1, was at 17 m depth relative to present sea level, and is marine in origin and Holocene in age. Maximum thicknesses of the two sedimentary deposits are:- Unit 2 = 8 m, and Unit 1 = 15 m.

INTRODUCTION Side-scan sonar and seismic-reflection profiling of the neashore adjacent to Port Elizabeth harbour are the basis of this geophysical report. Studies of a similar nature have been undertaken in the areas surrounding St. Croix Island and the Sundays River mouth, and reports covering these two areas are 38 currently in the process of being compiled. Two of the regions are under consideration for future development, viz., the expansion of existing harbour facilities at Port Elizabeth, and the construction of a shipyard/drydock at Jahleel Island. An iron-ore terminal was once planned for St. Croix Island, but it has now been proclaimed a marine nature reserve. The purpose of this report is threefold: (i) To provide detailed information on a small area within the bounds of a regional study of Algoa Bay. (ii) To allow comparison with detailed investigations being made at the St. Croix Island triad, and the Sundays River mouth. (iii) To supplement information on the existing Port Elizabeth docking facility with a view to future expansion.

METHODS AND EQUIPME T Side-scan-sonar images and seismic-reflection profiles have been obtained along 15 survey lines in the nearshore off Port Elizabeth harbour. The lines extend in a NW-SE direction between 10 and 30 m depth (Fig.l), and line coverage for the survey totals in excess of 100 km. With a line spacing of 350 m, and a side-scan range-setting of 200 m, imagery of the seafloor was obtained for an area covering approximately 30 km^ (Fig.2). Position fixing f the ship (not equipment streaming off its stern) was established with a master (onboard) and two slave (onshore) tellurometers at 1-minute time intervals. Sediment samples lying in close proximity to the study area and utilized in this investigation were collected during the regional survey of Algoa Bay. Equipment used on board ship incorporated the following: An EG&G side-scan sonar with model 259-3 recorder and model 272 tow fish; an 82 cm-* Bolt airgun pressurized by a Deutz/Ingersol-Rand 39 four-stage compressor? a 24-element single-trace array (built up from Aquadine Ql elements) ; a Krohn-hite model 3100 band-pass filter; a Keithly model 840 amplifier; an EPC model 4100 facsimile recorder; a Plessey 'Hydrodist' model MB 201 tellurometer system; and a modified Van Veen grab.

SIDE-SCAN SONAR

Plotting procedure The compression of side-scan imagery in the direction of ship's travel is a function of two independent variables viz., the speed of the fish through the water, and the preset paper- rate of the side-scan recorder. In this study, images were not corrected using distortion ellipses (Flemming, 1976) but rather, a track mosaic (Fig.l) was drawn from the ship's track (Fig.3) to eliminate image distortions in compiling an isometric map of the seafloor's acoustic response. Careful examination of the sonar records revealed that only three distinct 'sediment types' could be identified, and the boundaries between some of them particularly along the edges of the records, were not always distinct (Plate 3.1).

Results Information from sediment samples collected in the vicinity of the study area helped in classifying the sediment types identified on the sonar records. The location of samples is shown in Figure 2, and their properties are given in Tables 1 and 2. The following sediment types were recognized (Fig.4):

Weakly-reflective smooth sediment This material occupies roughly 80% of the study area and sonar records show it to be a smooth-surface deposit without 40 bedforms. This does not preclude the possibility of small-scale ripples being present because the resolution of the instrument is not sufficient to detect ripples commonly associated with fine­ grained sand. Note that the pseudo-ripples present on Line 5,0 (Plate 1) are in reality simply due to sea-surface noise. The sediment is inferred from the samples to be fine sand (Table 1) with progressively increasing amounts of mud in deep water towards the northeast (at about 30 m depth) Compositionally (Table 2), it consists nearly entirely of terrigenous and calcareous material (^55% and M5% respectively) close to shore, while in the deeper parts, terrigenous mud increases in abundance with consequent dilution of the calcareous component to only ^Sl.

Weakly-reflective undulating sediment The texture of this material is, in all probability, very similar to that described previously. The only significant difference noticeable on the sonar records is that the seafloor exhibits a degree of roughness not observable elsewhere. The overall reflectivity of the seafloor is, however, the same. The roughness- is expressed by small-scale undulations without orientation (see Lines 3,2 and 5,0, Plate 1), which is an aspect arguing against their formation by wave or current movement. The geographic location of this 'undulating' sediment-belt may have some bearing on its presence. First of all, an interesting fact is that it flanks the seaward edge of two 'lobes' of Holocene sediment (Fig.5) and that it peters out towards the north. Secondly, except for an area facing a coastal foreland located 4 km southeast of P.E. harbour, the belt of undulating sediment sub-parallels the trenri of the 20 m isobath in a region where the seafloor is relatively steep (Fig.6). Initially it was thought that the irregular undulations 41 represented small mounds and depressions caused by burrowing meiofauna, and were preserved in these regions because of a slackness in wave and current activity. It is now thought more likely (du Plessis; pers.comm.) that the undulations represent accumulations of shell-hash that have washed off the rocky bank located southeast of P.E. harbour, and have been carried northeastwards by longshore drift.

Strongly-reflective sediment These deposits are small, often elongate and sub-parallel t one another, and occur principally in the shallowest (M2 m) and deepest (^25 m) parts of the study area. Without access to samples of this material, it is tentatively suggested that the strongly-reflective signature is derived from gravel, probably mollusc shells, and that the linear disposition of the deposits is inherited from sandy bedforms like stringers and longitudinal dunes migrating across this gravelly storm-pavement. In other words, the strongly-reflective deposits are thought to be transient features which form in response to wave activity in shallow water, and to longshore or diffraction currents at greater depths.

Very strongly-reflective material These widely-scattered, small areas were considered, prior to examining the seismic records, to be isolated inliers of basement projecting through the Holocene sand-prism. With no seismic evidence to support this postulate, the strongly reflective areas are now considered to be solid anthropogenic detritus, such as ship remains, which rest directly on the seafloor.

SEISMIC-REFLECTION PROFILING

Plotting procedure The study area was not surveyed continuously in a single 42 session and operations had to be called off midway due to foul weather. Three different air-gun/ship/array configurations were used as a result, and this gave rise to problems in plotting the seafloor and sub-bottom surface depths. A set of curves was therefore drawn (du Plessis; pers.comm.), one for each configuration, in order to convert 2-way travel-time in milliseconds to actual depths in metres (accuracy estimated to be approximately 1 m). An overlay of each seismic profile (15 altogether) was drawn on tracing paper to illustrate sea level, the sea floor, two consistent sub-bottom surfaces (1/2 and 2/B), and as much basement structure as was evident. From these overlays two-way travel time of the three surfaces was read in milliseconds at 1- minute intervals and the values were then converted to metres using the previously described transformation curves. Depth values obtained in this manner were plotted onto charts of the ship's track (Fig.3) and hand-contoured at 1 m depth intervals. The thickness of various sedimentary units was derived by subtracting corresponding surface-depth values, and plotting and contouring this data. Altogether, three depth and three isopach maps were compiled as shown in Figure 7. Whereas the isopach maps are relatively unaffected by errors in measurement incurred whilst profile-reading, the three depth- maps exhibit extreme sensitivity because of the small vertical rates of change over long horizontal distances. An attempt was made to smooth the erroneous fluctuations in isopleth regularity by averaging the depth-values (from 3 to 9 points) contained within a 2 cm diameter circle (600 m on map sca'.e of 1:30 000). Contouring these new depth-values gave rise to maps which were essentially devoid of small-scale fluctuations, but still contained large-scale irregularities r-t uncertain merit. It was decided, therefore, to reject these 'smoothed' maps in favour of 43 the original ones which were based on absolute depth measurements.

Results An idealized section showing the horizons and geological units mapped from the seismic reflection data is presented in Figure 7. Each of these parameters will now be described in sequence starting with the oldest.

Basement Gerrard (1968) has shown in a preliminary seismic survey, that major onshore basement-structures trend into Algoa Bay, at least to within the study-area. Since the emphasis of this study was to evaluate Quaternary sedimentation in the vicinity of P.E. harbour, Palaeozoic Table Mountain Group (TMG) sandstone, together with Cretaceous Kirkwood Formation sediments and beds of the Tertiary Alexandria Formation, have al?. been grouped together under the heading 'basement'. The southern part of the Algoa Basin consists of a half- graben structure called the Swartkops trough. On its rorthern flank, the trough is truncated by the Coega fault, which is a normal fault with a throw of at least 1800 m (Shone, 1976). The geology of the Port Elizabeth surroundings (Geological Survey, 1962) indicated that TMG sandstone dips steeply northwards at 30° to 70° and is sometimes overturned. Unconformably above these highly-strained rocks is the Kirkwood Formation which Shone (op.cit., Fig.4) has dipping northwards at much shallower angles viz., 3° to 10°. Seismic-reflection profiles from the present investigation show sediments dipping northwards at a gentle angle as well, thereby suggesting that shallow basement in the study area may belong to the Kirkwood Formation. Minor folding of strata on profiles 3,0; 2,4; 3,6; 3,8 and 4,0 (ie., the western 44 part o£ the study area - see Fig.3) is probably the result of continued subsidence of the half-graben during Cretaceous times (Shone, 1976). Four representative seismic profiles are illustrated in Figure 8 and four are shown in Plate 2.

Depth and age of Surface 2/B (Fig.9) An erosion surface (2/B) was cut into basement prior to Quaternary sedimentary Unit 2 being laid down. Near P.E. harbour, the surface consists of a narrow, bifurcate valley that opens toward the southeast. Both tributaries of the valley join at about -34 m and swing eastwards into another valley, which is broad, and parallels thi northeastern edge of the study area. Geographically, the tributaries of the narrow valley can be extended backwards to intercept..the Baakens River and the Papkuils River and the broad, open valley to the east can be linked with the much more extensive Swartkops River. Without additional evidence to rely on, it is postulated that the three valleys represent drainage channels cut into basement during a Pleistocene sea-level lowering. Similar tentative conclusions have been derived from an investigation of pre-Holocene drainage channels that extend into the bay from the Sundays and Coega Rivers farther to the east (Bremner and du Plessis, 1980).

Thickness and age of Unit 2 (Fig.10) Unit 2, without any recognizable internal structure, fills the basement valleys described previously, but only along the upper reaches of the channels close to the present-day shoreline. In the deeper parts of the study area, Unit 2 is thin or entirely absent, and only adjacent to the coastal foreland 4 km southeast of P.E. harbour does it appear to any extent. Here, the sediment (7,7 m thick) has accumulated, not in a basement depression as 45 with the river valleys, but on an even surface that dips steeply towards the northeast away from shore (Fig.9). 3oth the channel- fill deposits in the northern part of the study area, and the 'pile' deposit in the south reach maximum thickness values of 6 to 8 m at, or shoaler than 25 m relative to Surface 2/B, and thins rapidly between 25 and 30 m to only 2 or 4m. The 30 m depth is significant in that it corresponds with sea level during the Wurm Interstadial (30 to 50 Ka; Tankard, 1976). For reasons expounded on in the next section. Unit 2 is believed to have accumulated at this time and not during the Holocene. The source of this material was clearly the coastal stretch from Shelly Bay (lying 30 km to the west of Cape Recife) to Cape Recife (Trigonometrical Survey, 1972) as aerial photographs of this coast show vast tracts of veg^taced dunes stretching across the peninsula and terminating at the coast between P.E. harbour and Cape Recife. An enormous quantity of aeolian sand must have accumulated above the 30 m isopleth on Surface 2/B (Fig.9) ie., on the backshore, whereas today, mobile, unvegetated dune-sand is limited to a narrow belt extending from 3 bay lying 2 km west of Cape Recife, to just north of the Cape. Sand carried from the backshore into the bay by what then must have been a west-souchwesterly prevailing wind, was soon dispersed through wave and current action, and hence the scare;.y of unconsolidated sediment below 30 m depth. Sediment composing the foreshore however, would have been transported northeastwards by littoral currents to the eroded remnants of a barrier beach that extends southeastwards from St.Croix Island (du Plessis; pers.comm.).

Depth and age of Surface 1/2 (Fig.11) At the end of the Wúrm interglacial, sea level regressed to about -120 m during Wurm II (Tankard, 1976). Evidence from 46 various parts of the world suggest that increased during the pleniglacials (Van Zinderen Bakker and Butzer, 1973; Schrader, 1979), and a climatic change such as this would have instituted vegetation and stabilization of dune-sand on the Wurm Interstadial bcckshore. At the same time, the deposit would have been incised by fast-flowing rivers, and Surface 1/2 exhibits altered, or in sore cases completely new river courses - this is the reason for expounding a Wiirm Interstadial age for Unit 2. In the deeper parts of the study area, ie., below -30 m, the sediment cover is thin or entirely absent, and the isopleth contours of Surface 2/B and 1/2 are virtually identical. Only at the sediment pile adjacent the coastal foreland 4 km south of P.E. harbour are the isopleths distinctly different. With the onset of the Flandrian transgression, vegetated and partially consolidated aeolian sediment on the backshore (above 23 m on Surface 1/2) was available for redistribution on the shelf. At two locations within the study area however, Unit 2 was able to withstand the rigours of the advancing surf-zone due to geomorphic peculiarities in the immediate neighbourhoods. One of these regions was the valley-and-ridge province to the north, and the other was adjacent to the coastal foreland located 4 km southeast of P.E. harbour. In the former area, Unit 2 was protected from the transgressing Flandrian shoreline in fluvially-eroded basement valleys, and in the latter area, a rocky island (now a submerged bank rising to -8 m, and situated 1 km northeastward of the coastal foreland; Bremner, 1979; Fig.4.1) served as a wave-diffraction centre, thereby ensuring Unit-2 preservation in its lee.

Thickness and age of Unit 1 (Fig.5) Like Unit 2, no internal structures could be identified in Unit 1 from seismic profiles. The locus of sedimentation of Unit 47

1 is, however, quite different to that of Unit 2, and the supply of material, and its mode of deposition was therefore entirely novel. Maximum thickness of Unit 1 (15,1 m) occurs at approximately -17 m (relative to the present seafloor; Fig.6), in two main lobes of sediment, and it is clear that iittle additional material was introduced to the environment because of the stabilized condition of the Wiirn-Ir.terstadial dunes. Besides the two areas where Unit 2 was afforded protection, the great bulk of Unit 2 was removed from the Wurm Interstadial backshore through wave-induced bottom currents, and deposited in the central part of the study area as Unit 1. Valleys eroded into Unit 2 during the WClrm II pleniglacial were filled by 6 or 7 m of Unit 1 sediment and the intervening ridges were covered by 1 or 2 m, thus giving rise to a smooth, gently-sloping seafloor with two very shallow valleys (Fig.6). Only 1 or 2m of Unit-1 material was deposited on top of Unit 2 in tue protected northwest- southwest side - hence a smoothly-sloping se-floor surface towards the northeast (Fig.6). An isopach map has been constructed to show total sediment accumulation from Wiïrm Interglacial times to the present ie., Unit 1 plus Unit 2 overlying basement (Fig.12). The overriding distribution is similar to that of Unit 1 simply because of the great thickness of sediment deposited in the irri d-r r-g ions of t ' e study ar^a during the Flandrian transgression.

The sea floor The depth to the sea floor compiled from seismic records (Fii.6) compares favourably with a bathymetric map of the area drawn from S.A.N, fairchart FC62 and S.A.R. chart PEH 51/F486. Considering the gentleness of the seafloor gradient, and the time elapsed between different surveys (seismics 1976; S.A.N,

fairchart A963; S.A.R. chart 1951), the maximum vertical 48 deviation is only about 1 m in the shoaler parts of the study area, and about */2 m in the deeper parts. The gradient of the sea floor increases rather abruptly from 0,07° (or 1 in 840) at 17 m depth, to 0,29° (or 1 in 200) farther offshore. This 17 m depth-level is regarded as wave base for present-day storm waves and indirect evidence to support this contention is the fact that the locus of maximum thickness of Unit 1 is also at -17 m. (Both these aspects viz., the change in slope of the sea floor, and the locus of maximum thickness of Unit 1 are evident in Lines 3,8 and 5,0 of Plate 2).

CONCLUSIONS (i) Sonar imagery has shown that approximately 80% of the sea floor has a weakly-reflective, smooth acoustic-response. The sediment is predominantly fine sand consisting of terrigenous and calcareous detritus (^55% and M5% respectively). (ii) Similar material, but with an undulating surface, skirts two 'lobes' of Holocene sediment at ^20 m depth, and also flanks the northern edge of a rocky bank located approximately 4 km southeast of P.E. harbour. The undulations are probably the result of shell-hash being swept off Rocky Bank through wave turbulence, and carried northeastwards by longshore currents. (iii) Small, elongate, strongly-reflective sedimentary deposits on the sonographs are interpreted as being due to a gravelly shell-layer which is partially buried beneath migrating sand-stringers and longitudinal dunes. The deposits are restricted to the shallower and deeper parts of the study area where wave and current activity respectively, are strongest. (iv) Seismic profiling has indicated a fluvially-incised 49

basement-surface, probably on sediments of the Kirkwood Formation. (v) Unit 2, which is aeolian-derived from bays and coves lying to the west of Cape Recife, was deposited during the Wurm Interglacial on the backshore. With a sea level at about -30 m, sediment carried into the bay was soon dispersed by littoral currents. (vi) Precipitation increased during the Wurm II pleniglacial causing vegetation of the aeolian dunes and incisement of Unit-2, often along new courses. (vii) Unit 2 survived the Flandrian transgression in two areas due to local geomorphic peculiarities. One of the areas was the fluvially-eroded valley-and-ridge province in the northern part of the study area, and the other was in the lee of Rocky Bank located approximately 4 km southeast of P.E. harbour.

(viii) The advancing surf zone saw reworking and erosion of most of Unit 2, with consequent transport and deposition of the bulk of this material to the central part of the study area (locus of Unit 1 at -17 m depth). Maximum thicknesses are: Unit 2 = 8 m; Unit 1 = 15 m. (ix) A change in slope is evident in the present-day sea fioi,r at -17 m which is attributed to wave base during pre: out day storm conditions.

ACKNOWLEDGEMENTS Mr. A. du Plessis was instrumental in calculating and plotting the transformation curves used to convert 2-way time in milliseconds to actual depths in metres. He is also to be thanked for carefully scrutinizing an early draft of this report.

REFERENCES

Bremner,J.M. 1979. The bathymetry of Algoa Bay. Rep. Dept.. of Mines, qeoi. .mrv. S. Air. 1980-0007:1-13. 50

Bremner,J.M. and du Plessis,A. 1980. Basement morphology and unconsolidated sediment in Algoa Bay. Rep. Dept. of Wines, geol. Surv. S. Afr. 1980-0206:1-7.

Flemming,B.W. 1978. Underwater sand dunes along the southeast African continental margin - observations and implications. Mar. Geol. 26:177-198. Geological Survey. 1962. Geological map of Port Elizabeth (3325D) and Alexandria (3326C). Dept. of Mines, S. Afr. Gerrard,I. 1968. Report on the geology of Algoa Bay. Prog. Rep. Agul. Bank geophys. Surv., Univ. cf Cape Town. 1-10. Schrader,H-J. 1979. Quaternary paleoclimatology of the Black Sea Basin. Sedim. Geol. 23:165-180. Shone,R.W. 1976. The of the Mesozoic Algoa Basin. Unpubl. M.Sc. thesis, Univ. of-Port Elizabeth. 1-48. Tankard,A.J. 1976. Pleistocene history and coastal morphology of the Ysterfontein- area. . Ann. S. Afr. Mus. 69:73-119. Trigonometric Survey. 1972. Topographic map of Port Elizabeth (3324) . Van Zinderen Bakker and Butzer,K.W. 1973. Quaternary environmental changes in southern Africa. Soil Science 116:236-248. Table 1 SEDIMENT MASSES IN AND AROUND ALGOA BAY

Area Volume Nass Region (sq. km) (cub.metres X IB6) (metric tons X in6)

ALGOA BAY I 7fl5 3P18 1K232 II 399 1836 4921 III 573 3297 8835 IV 431 224C 6003 V 353 2839 7608 VI 298 183« 49C5

Total 2759 1586C 425P4

COASTAL DUME: Total 127 A2C9 11E13 Table 2 LLUVIAL INPORT OE SEDIKENT IMTC ALGCA BAY

IUver Catchment area Effective annual Effective annual (sq. krrO sediment yield bedload yield (netric tons x ]Pr>) (metric tons x ]Pr')

Swartkops 25"C 3,P (!, 2 Sundays ?1?C ?2,P I,?

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-87«

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Plate 2 51

VIII THE AGULHAS PLATEAU, SOUTH ATLANTIC OPENING, AND RIDGE- JUMPS SOUTH OF THE AGULHAS FALKLAND FRACTURE ZONE by A.K.Martin, C.J.H.Hartnady, D.B.Murray

ABSTRACT Mesozoic sea-floor spreading anomalies in the Natal Valley constrain plate tectonic evolution south of the Agulhas Falkland Fracture Zone (AFFZ) and the formation of the oceanic part of the Agulhas Plateau. Anomalies M10-M3 in the Natal Valley are offset H300 km by the Agulhas transform fault from their equivalents in the extreme southern Cape Basin. A series of computed palaeopositions show that the offset of M300 km was maintained from break-up until M0 times. By anomaly 34, the offset was vL270 km. Thus only slight asymmetry or a ridge-jump of ^30 km occurred in the Cretaceous Quiet Zone. It is unlikely that the northern oceanic patt of the Agulhas Plateau - 175 km wide - was caused by a ridge-jump. The Islas Orcadas Rise, Meteor Rise and northern Agulhas Plateau were formed simultaneously on thickened oceanic crust 97.3 - 90.3 Myr old. Continued sea-floor spreading between 90 and 67 Myr split the elevated feature in two - Islas Orcadas and Meteor Rise to the west, Agtilhas Plateau to the east. When a well documented westward ridge-jump occurred ^67 Myr ago, the new ridge centred on the western half of the original feature, split it and separated the Islas Orcadas Rise from the Meteor Rise. The new accreting ridge associated with the ridge- jump did not necessarily create an elevated feature, but rather jumped to pre-existing elevated crust. An alternative spreading Model is proposed for the Mozambique Ridge, the continental Agulhas Plateau and the Burdwood Bank: Agulhas Plateau drifted to its present position between 127 and 108 Myr ago and was rifted from Burdwood Bank 52

105 Myr ago.

AGULHAS PLATEAU The Agulhas Plateau is a large aseismic bathymetric high lying between 250 and 950 km south of South Africa. It is separated from the African margin by the and is flanked by the Agulhas and Transkei Basins (Fig.l). Its central region is 2000-3000 m deep, rising about 2500 m above the surrounding abyssal areas. A northern triangular shaped area of rough relief is offset to the north-east from a southern rectangular shaped portion of smoother relief. Regarding the nature of its underlying crust, Scrutton (iy/J) suggested tnat it comprises oceanic crust of an abandoned spreading centre. Through seismic refraction and magnetic modelling, Barrett (1977) showed that the northern region is underlain by thickened oceanic volcanic basement. Barker (1979) suggested the excess volcanism of the northern Agulhas Plateau represents the site to which an accreting ridge jumped in tne Cretaceous Quiet Zone. Recently Tucholke et al. (1981) proposed a continental origin for the southern area characterised by smooth basement and an oceanic origin for the northern area of rough basement. A layer of seismic velocity 5.8 - 6.4 km/sec (minimum thickness 4.3 - 7.7. km) represents a 'granitic layer' indicating continental crust. This interpretation is supported by dredge-hauls of quartzo-feldspathic gneiss of lower amphibolite to granulite which give isotopic dates of 498 ±-17 and 1105 t-36 Myr (Allen & Tucholke, 1981). Continental crust is flanked to the west by a narrow south-west trending strip of thickened oceanic material (Fig.l), and to the north­ west the crust is intermediate between oceanic and continental in character. Patches of south-east trending irregular basement may represent thickened oceanic material intruding continental 53 basement.

Plate Tectonic Reconstruction Prior to the break-up of Gondwanaland the northern escarpment of the continental Falkland Plateau fitted against the sheared Agulhas margin of south-eastern Africa, with its eastern tip extending into the Natal Valley (Scrutton, 1973; Rabinowitz and LaBrecque, 1979; Martin et al., 1981). The northern Agulhas Plateau lies in the sea-floor spreading compartment between the Falkland Plateau and the Natal Valley (F.P.N.v.). Its oceanic nature presents no obstacle to the above reconstruction. The southern continental part of the Agulhas Plateau lies in the spreading compartment south-west of the Mozambique Ridge (M.R.).

Sea-floor spreading in the Natal Valley Mesozoic sea-floor spreading anomalies M10-M0 have recently been identified in the Natal Valley between the southeastern African margin and the Mozambique Ridge (Goodlad et al., 1982; Martin et al., in press). The anomalies are oriented NW-SE perpendicular to the sheared Agulhas Margin and exhibit similar spreading rates to previously recognised Mesozoic sequences in the South Atlantic and off the Falkland Plateau in the Georgia Basin (Larson and Ladd, 1973; Barker, 1979; LaBrecque and Hayes, 1979; Rabinowitz and Labrec'-jue, 1979). The Natal Valley anomalies are offset vL300 km to the north-east from their equivalents in the southern Cape Basin. This is an improved estimate of the original offset along the Agulhas Falkland Fracture Zone (AFFZ).

Palaeopositions of the Falkland Plateau By superimposing conjugate sets of Natal Valley and Georgia Basin magnetic anomalies, we have computed palaeopositions for 54

South Africa relative to Africa. For example, a reconstruction for anomaly M2 shows that Cape and Argentine Basin anomalies are effectively juxtaposed when Natal Valley and Georgia Basin anomalies are reconstructed on a fossil accreting ridge (Fig.2). Reconstructions carried out using anomalies M10, M4 and M0 achieved similar results (Martin et al., in press summarised in Fig.4). Note that in the Natal Valley (Fig.l) anomalies have not been recognised east of 33°E and we are uncertain of the nature of the crust in the region 33 - 35° £, 29 - 31°ó. However, the most southerly identification of M0 in the Georgia Basin (Fig.3) indicates that by that time the accreting ridge extended across the full width of the F.P.N.V. spreading compartment. By anomaly

34 Lime Liie i idge extended acioi-s boLh IÍIH F.r.w.V. arid n.R. spreading compartments.

Ridge-Jumps in the Falkland Plateau Natal Valley (F.P.N.V.) Compartment The present-day offset along the Agulhas Falkland Fracture Zone (AFFZ) is appreciably shorter than the original offset indicating that phases of asymmetric spreading or westward ridge- jumps have occured. du Plessis (1977) discovered a fossil ridge centred on anomaly 26, and estimated the associated westward ridge-jump to be 850 km. Assuming an original offset of only 1130 km, he considered this jump sufficient to reduce the offset to the present-day value given as 280 km. Barker (1979) suggested that two ridge-jumps between anomalies 28 and 27 accounted for a westward move in the spreading centre of 800 km. He proposed that these jumps caused elevated crust at the site of the new spreading centre in the Meteor Rise area (Barker's elevations W and X). Estimating the original offset as 1400 km and the present-day one as 200 km, Barker calculated that the ridge must have jumped a total of 1200 km to the west. Only 800 55 km is accounted for by the above jump, pointing to the possibility of a 400 km ridge-jump in the Cretaceous Quiet Zone. Such a jump causing an elevated area of oceanic crust has beep, invoked as the origin of the Northern Agulhas Plateau (Barker, 1979; Tucholke et al., 1981). Interpreting the magnetic data differently, LaBrecque and Hayes (1979) place anomaly 33 farther from anomaly 34 south-west of the Agulhas Plateau. This implies asymmetric spreading between anomalies 34 and 31, reducing the offset by 280 km. In conjunction with a discrete ridge-jump of 825 km, the offset is reduced by a total of 1105 km between anomalies 34 o.nd 25. This interpretation is based on a larger data set and is corroborated in part by complementary surveys of Bergh and Barrett (1980). LaBrecque and Hayes estimated the original offset as 1400 km, and noted that 1105 km of westward movement of the ridge is only 55 km less than needed to produce the present-day offset given as 240 km. Clearly, existing estimates of the original and present-day offsets vary. Natal Valley (Martin et al., 1981) and Cape Basin data (Gerrard and Smith, 1980) indicate that the original offset along the AFFZ was ^1300 km. Successive reconstructions (e.g. Fig.2) show that within the limits of navigational accuracy the offset of ^1300 km remained constant from break-up until M0 time. By anomaly 34 times the offset was ^1270 km (Fig.3). Moreover Natal Valley M0 crust is 1180 km northeast of anomaly 34 crust in the , suggesting a total of 2360 km of crust was produced in this period. This agrees very v.e.1 with the value of 2350 km predicted by Barker (1979) from a consideration of spreading rates. Thus only a slight asymmetry or ridge-jump ( .30 km maximum) occurred in the Cretaceous Quiet Zone. As the offset at anomaly 34 time was <\,1270 km, the reduction in offset of 1105 km (LaBrecque and Hayes, 1979) would have decreased the offset to 165 km. This is 35 - 75 km less than the 200 - 240 km previously 56

estimated for the present-day offset (Barker, 1979; LaBrecque and Hayes, 1979). New magnetic profiles run more closely to the AFFZ than previously, indicate that the present-day offset is in fact only 170 km (Murray, in prep.). It is unlikely therefore that the northern oceanic part of the Agulhas Plateau, which is up to 175 km wide, was caused by a ridge-jump. Dating M0 and anomaly 34 as 108 and 80 Myr respectively (Larson and Hilde, 1975; LaBrecque et al., 1977) the average half spreading rate in the Cretaceous Quiet Zone is 4.2 cm/yr. Using this, oceanic crust of the Agulhas Plateau, lying between 450 and 725 km from M0, is dated at 97.3 - 90.7 Myr, while the central area is dated at 93.0 Myr (630 km fron-, M0) . These dates agree well with micropalaeontological data which suggests basement of the Agulhas Plateau is Cenomanian (>95 Myr)(Tucholke et al., 1981). When the accreting ridge jumped around anomaly 25 - 29 time, the new spreading ridge was centred on a bathymetric high (Barker, 1979; LaBrecque and Hayes, 1979). Through continued sea-floor spreading the elevated feature was split and separated. The western half, the Islas Orcadas Rise now lies in the Georgia Basin and the eastern half, the Meteor Rise lies in the Agulhas Basin (Figs.5&6). The elongate Islas Orcadas Rise trends 30°W of north and is not parallel to M0 crust in the Georgia Basin. At the northern end of the F.p.N.V. spreading compartment it centres on crust 600 - 650 km from M0. Its western and eastern flanks are respectively -v525 and -v700 km from M0. It lies on 95.5 - 91.3 Myr old crust and, in this respect is very similar to the oceanic Agulhas Plateau. Similarly at their northern ends, the •young' flanks of the Meteor rise and Agulhas Plateau are both ^50 km distant from anomaly 34, and both are centred on crust VS20 km from anomaly 34. It appears that one phase of excessive volcanism 97.3 - 90.7 Myr ago produced an elevated feature on the 57 accreting ridge encompassing Islas Orcadas Rise, Meteor Rise and Agulhas Plateau. Today the crust of these areas is 2000 m above crust of the surrounding ocean basins (Barrett, 1977; LaBrecque and Hayes, 1979). Assuming subsidence according to the model of Parsons and Sclater (1977) the elevated crust was originally ^500 m dee^ with isolated areas exposed subaerially. Continued sea- flocr spreading split the elevated area in two, with the Islas Orcadas and Meteor Rises to the west and Agulhas Plateau to the east. When the ridge jumped around anomaly 25 - 29, the new spreading ridge was centred on the elevated crust produced 30 Myr earlier. By this time it had subsided to about 2500 m depth. Thus the western half of the original feature was in turn split in half and spreading from anomaly 25 to the present, separated the Islas Orcadas and Meteor Rises. During the second splitting phase the elevated crust was reheated and uplifted and then gradually subsided to its present depth as it moved away from the spreading ridge. This may or may not have been accompanied by a second phase of excess volcanism. Thus the newly established spreading centre associated with the ridge-jump need not necessarily have caused the elevated feature (Barker hypothesis, 1^79); rather, it jumped to pre-existing elevated oceanic crust.

An Alternative Break-Up Model No sea-flcoi spreading anomalies have been identified between the Agulhas Plateau and the Mozambique Ridge to constrain break-up in this region. Tucholke et al. (1981) suggested that as sea-floor spreading proceeded in the Natal Valley and the south Atlantic 127 - 108 Myr ago, a spreading centre formed oceanic crust south-west of the Agulhas Plateau. When the Natal Valley spreading centre reached the southern end of the Mozambique Ridge it extended southeastwards separating the Agulhas Plateau and the Mozambique Ridge. At this stage in their 58 model, spreading centres existed on both sides of the Agulhas Plateau. These jumped to the Agulhas Plateau intruding it and extending it. We have assumed that the Burdwood Bank has been rigidly attached to the Falkland Plateau throughout the opening of the South Atlantic. At the South American margin of Gondwanaland, of oceanic crust formed an active volcanic arc in the Middle and Late Jurassic (Dalziel et ai., 1974; De Wit, 1977; De Wit and Stern, 1981). At this time South Geoigia Isiand was adjacent to the southern side of Burdwood Bank (Winn, 1978). Silicic volcanism on South Georgia and in Patagonia has been dated at 180 - 200 and 157 - 163 Myr respectively (Tanner and Rex, 1979; De Wit, 1977; Herve et al., 1981). A back arc basin opened on the continental side of the arc at least before 118 Myr ago (Dalziel et al., 1974) and after the silicic volcanic episode. In South Georgia mafic rocks of the back-arc are cut by 127 Myr tonalites and initial emplacement of mafic material is dated at M40 Myr. Silicic rocks of the Tobifera Formation are encountered east of Tierra del Fuego, and Burdwood Bank and North Scotia Ridge are considered part of this remnant arc (Ludwig et al., 1979). Purdwood Bank is thus a pre-drift feature, since South Atlantic opening began 122 - 127 Myr ago (Martin et al., in press). In its pre-drift position Burdwood Bank lay near the Mozambique Ridge with the gap between filled by Agulhas Plateau. As sea-floor spreading began in the Natal Valley, the Aguliias Plateau, attached to Burdwood Bank rifted away from the Mozambique Ridge region. (At this stage it is unclear whether the Mozambique Ridge is oceanic or continental in character. Therefore, it can only be said that Agulhas Plateau rifted away from continental crust occupying the Mozambique Ridge region.). Spreading ridges in the F.P.N.v. and M.R. compartments were 59 subparallel and offset by 350 km. Between 108 and 105 Myr ago (M0 and pole change reconstructions) Agulhas Plateau reached its present position relative to Africa. The M.R. spreading ridge then jumped -x.500 km to the west, rifting apart Agulhas Plateau and Burdwood Bank. South-east trending areas of thickened oceanic crust were emplaced ir. conjunction with south-east oriented faulting (Tucholke et al., 1981 Figs.2 and 6). This event may have been associated with the change in opening pole of the South Atlantic (Fig.7). F.P.N.v. and M.R. spreading ridges were offset by ^850 km. Through continued sea-floor spreading the F.p.N.V. ridge reached the north-east corner of the continental Agulhas Plateau. Excess volcanism ensued as the ridge encountered high residual heat flow resulting from rifting of the Agulhas Plateau 5-10 Myr earlier. A slight asymmetry favouring the African side may have occurred. South-west trending faults and east-west bathymetric trends resulted from the interaction of ridge and active transform fault (c.f. Searle, 1979; Schouten et al., 1980). The offset of the northern and southern parts of the Agulhas Plateau is associated with the transform fault and westward rafting of the western half of the thickened oceanic crust. The excess volcanism of the Agulhas Plateau region began just after the Falkland Plateau cleared the southern tip of Africa, and this may have led to some plate re­ organization. Before anomaly 34 times the M.R. ridge jumped back, perhaps to the western edge of the Agulhas Plateau around 90 - 95 Myr as the ridge in the F.P.N.V. compartment extruded excess material. At this later stage a triple junction was formed a»- a time when global plate re-organizacion occurred (Johnson et al., 1980). The south-west spur of the Agulhas Plateau evolved as the triple junction migrated south-west. The south-east section of Meteor Rise and possibly the Maud Rise off Antarctica were formed in the same process. From its position 60

relative to anomaly 34 (Fig.l and Norton and Sclater, 1979) the oceanic part of the Madagascar Ridge (Sinha et al., 1981) may also have formed during this phase of plata re-organization and leaky volcanism.

DISCUSSION Sea-floor spreading anomalies in the Natal Valley place constraints on the origin of the oceanic northern Agulhas Plateau. No significant ridge-jump occurred prior to anomaly 25 - 29. The northern Agulhas Plateau does not represent the site of a ridge-jump. The site to which the ridge jumped at anomaly 25 - 29 was a pre-existing bathymetric high and was not necessarily caused by the jump. LaBrecque and Hayes (1979) suggested that some 200 km of the "Malvinas Plate" was subducted at the Georgia Rise. The symmetry exhibited by Meteor Rise, Islas Orcadas Rise and Agulhas Plateau mitigates against substancial subduction of oceanic crust in this region. The Mozambique Ridge (M.R.) spreading compartment lacks identified spreading anomalies. Thus both the above break-up model and that of Tucholke et al. are speculative and complicated. Ludwig (quoted in Tucholke et al., 1981) suggested that oceanic crust formed in the basinal area of the Falkland Plateau. Extension during this episode allowed south-east oriented oceanic crust to upwell in the Agulhas Plateau (Tucholke et al., 1981). The Falkland Plateau Basin and the Outeniqua Basin of South Africa were once a continuous feature which formed in the Middle- Late Jurassic (Barker, Dalziel et al., 1977; Martin et al., 1981). The maintenance of a 1300 km offset along the AFFZ from break-up until M0 time indicates that any extension of the Falkland Plateau and possible emplacement of oceanic crust stopped before the South Atlantic opened. Although this 61 mechanism may have caused tension on the Agulhas Plateau, before the South Atlantic sea-floor spreading phase, a ridge-jump of a south-east oriented ridge is as plausible a mechanism for emplacing south-east oriented crust on the Agulhas Plateau.

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Barker,P.F. 1979. The history of ridge-crest off-set at the Falkland Agulhas Fracture Zone from a small-circle geophysical profile. Geophys. J. R. Astr. Soc. 59:131-145.

, Dalziel,I.W.D. et al. 1977. Init. Rep. Deep Sea Drill. Proj. 36. Washington D.C. (U.S. Govt.Print.Of f.) .

Barrett,D.M. 1977. The Agulhas Plateau : a marine geophysical study. Geol. Soc. Am. Bull. 88:749-763.

Bergh,H.W. and Barrett,D.M. 1980. Agulhas Basin magnetic bight. Nature 287:591-595.

Dalziel,I.W.D., De Wit,M.J. and Palmer,K.F. 1974. Fossil marginal basin in the southern Andes. Nature 250:291-294.

De Wit,M.J. 1977. The evolution of the Scotia Arc as a key to the reconstruction of South-western Gondwanaland. Tectonophysics 37:53-81.

and Stern,C.R. 1981. Variations in the degree of crustal extension during formation of a back-arc basin. Tectonophysics 72:229-260. du Plessis,A. 1977. Sea floor spreading south of the Agulhas Fracture Zone. Nature 270:719-721.

Gerrard,!. and Smith,G.C.(1980). The post Palaeozoic succession and structure of the south-western African continental margin. Southern Oil Expl. Corp. Kept.

Goodlad,S.W., Martin,A.K. and Hartnady,C.J.H. 1982. Mesozoic magnetic anomalies in the southern Natal Valley. Nature 295:686-688.

Herve,F., Nelson,E., Kawashita,K. and Suarez,M. 1981. New isotopic ages and the timing of orogenic events in the Cordillera Darwin, southernmost Chilean Andes. Earth Planet. Sci. Letts. 55:257-265. 62

Johnson,B.D., Powell,C.McA. and Veevers,J.J. 1980. Eaily spreading history in the Indian Ocean between India and Australia. Earth Planet. Sci. Letts. 47:131-143. LaBrecque,J.L. and Hayes,D.E. 1979. Seafloor spreading history of the Agulhas Basin. Earth Planet. Sci. Letts. 45:411-428. , Kent,D.V. and Cande,S.C. 1977. Revised magnetic polarity time-scala for the Late Cretaceous and Cenozoic time. Geology 5:330-335. Larson,R.L. and Ladd,J.W. 1973. Evidence for the opening of the south Atlantic in the Early Cretaceous. Nature 246:209-212. and Hilde,T.W.C. 1975. A revised time-scale of magnetic reversals for the Early Cretaceous and Late Jurassic. J. Geophys. Res. 80:2586-2594. Ludwig,W.J., Windisch,C.C., Houtz,R.E. and Ewing,J.I. 1979. Structure of Falkland Plateau and off-shore Tierra del Fuego, Argentina. In: Geological and geophysical investigations of continental margins. Am. Ass. Pet. Geol. Mem. 29:125-137. Martin,A.K., Hartnady,C.J.H. and Goodlad,S.W. 1981. A revised fit of South America and South Central Africa. Earth Planet. Sci. Letts. 54(2):293-305. , Goodlad,S.W., Hartnady,C.J.H. and du Plessis,A. (in press). Cretaceous Palaeo-positions of the Falkland Plateau relative to South Africa using Mesozoic sea-floor spreading anomalies. Geophys. J. R. Astr. Soc. 71. Parsons,B. and Sclater,J.G. 1977. An analysis of the variatijn of ocean floor bathymetry and heat flow with age. J^ Geophys. Res. 82:803-827. Rabinowitz,P.D. and LaBrecque,J.K. 1979. The Mesozoic South Atlantic Ocean and evolution of its Continental Margins. J. Geophys. Res. 84:5973-6002. Schouten,H., Karsov.J. and Dick,H. 1980. Geometry of transform faults. Nature 288:470-473. Scrutton,R.A. 1973. Structure and evolution of the sea-floor south of South Africa. Earth Planet. Sci. Letts. 19:250- 256.

Searle,R.C. 1979. Side-scan sonar studies of North Atlantic fracture zones. J. Geol. So£. Lond. 136:283-292. Sinha,M.C, Louden,K.E. and Parsons,B. 1981. The crustal structure of the Madagascar Ridge. Geophys. J. R. Astr. Soc. 66:351-377. Tanner, P. W.G. vi\a Uex,D... i'j • .K Timing oi ev?nts in an Early Cretaceous island arc-T-jrqir.al basin system on South Georgia. C^ol. Han. ) if. -. ' 6 "'-?4f .

Tucholt"-',B.fc. , >•• i;t ? , p. ••...... •„•• - ' \ , p . " .'•?'. Continental crust beU'-M:. ' ; • '.-. Am. : !.-i.. ' :•'•<-, ,<:•. >;• •• r Intlun Ocean. 63

WinnrR.D. 1978. Upper Mesozoic Flysch of Tierra del Fuego and South Georgia Island : A sedimentological approach to lithosphere plate restoration. Geol. Soc. Am. Bull. 89:533- 547. FIGURE CAPTIONS

Fig.l Summary of known magnetic lineations in the south-west Indian Ocean, (refs in text). Crustal types of Agulhas Plateau - Tucholke et al., 1981.

Fig.2 Reconstruction for anomaly M2. 50.50°N, 34.16°W, 53.42° rotation South America relative to Africa. Argentinian margin and Falkland Plateau outlined by 3000 m isobath and shaded.

Fig.3 Anomaly 34 reconstruction. 64.84°N, 38.01°W, 33.9° rotation. (After Bergh •& Barrett, 1980). Symbols as for Fig.2. Fig.4 Palaeopositions of the Falkland Plateau from pre-drift to anomaly 34. Fuji diagrams and rotation parameters from Figures 2 and 3 and Martin et al., (in press). Pole change reconstruction (Rabinowitz and LaBrecque, 1979) is dated at 105 Nyr, using average spreading rate of 4.2 cm/yr in the Cretaceous Quiet zone.

Fig.5 The relative positions of the continents, oceanic highs and magnetic anomalies for the present day.

Fig.6 Anomaly 25 refit (LaBrecque & Hayes, 1979) showing Islas Orcadas and Meteor Rises just after the ridge-jump.

Fig.7 Pre-drift, M0 and pole-change reconstructions of the Falkland Plateau relative to the Agulhas Plateau (shown in its present position relative to Africa), Burdwood Bank and North Scotia Ridge oultined by 3000 m isobath. TT 20 IF

oceanic crust

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IX BATHYMETRY OF THE INNER SHELF ALONG THE CAPE WEST COAST BETWEEN THE ORANGE RIVER AND PORT NOLLOTH by R.H. De Decker

ABSTRACT The bathymetry between the Orange River and Port Nolloth is important as it contains aspects of geomorphic features found along the rest of the South African west coast. Principal features recognized are: 1) the Orannu River delta; 2) tha rugged inner shelf extending down to 4C m; 'i) the inner-shelf edge; and 4) the gently dipping mudbelt offsnoie trom the inner shelf. In addition the bathymetry reflects the variable lithologies along the coastline, the presence oi a possible previous sea level at between 30 m and 40 m below sea level, the pathways of sediment movement and probable depo-centres.

INTRODUCTION As part of a newly-defined West Coast Project, the Joint Geological Survey/University of Cape Town Marine Geoscience Unit is undertaking a detailed geophysical survey of the inner shelf between the shore and a depth of about 100 m. Emphasis is placed on complete coverage by side-scan sonar and continuous seismic profiling, using Klein and £G and G side-scan sonars, and an EDO- Western 3,5 kHz sub-bottom profiler. Follow-up work will involve sediment sampling using a grab, divers, and a vibrocorer. Initially the concession areas controlled by State Alluvial Diggings will be surveyed, but the aim is to cover the entire west coast south of the Orange River. To date concession areas No.l (Orange River to Wreck Point) and No.3 (Cliff Point to just south of Port Nolloth) have been completed (Fig.l). The bathymetry between Port Nolloth and the Oiange River is 65 important since features that have been recognized (O'Shea 1971, Rogers 1977, Birch 1975) along the rest of the west coast are particularly well developed in this area. A cursory investigation of the bathymetry reveals four geomorphic zones between the shoreline :nd the 120 m isobath (Fig.l): 1) the submerged Orange River delta; 2) a rugged inner shelf, which is well developed south of Homewood Harbour and extends to 40 m below sea level; 3) the edge of the inner shelf between the 40 m and 70 m isobaths; and 4) the smooth, more gently sloping zone between 70 m and 120 m - the Orange River mud belt (Rogers 1977).

PREVIOUS WORK The discovery of diamonds along the west coast of soutnern Africa almost 60 years ago focussed considerable attention on this region, mainly onshore. Only in the past 20 years, however, has attention been given to offshore areas. Exploration of the inner shelf began with emphasis being directed to the area north of the Orange River (Hallam 1964, Hoyt et. al^ 1969, Murray 1969, Murray e_t a_l 1970, Fowler 1976) . Other workers have made regional studies of parts of the Cape west coast (Ahmed 1968, O'Shea 1971, Birch 1975, Rogers 1977). Within the study area, O'Shea (1971) has produced a 1:100 000 scale map showing isobaths and sediment isopachs at 20 ft (6,1 m) intervals, and Rogers (1977, Figs.11-18, 11-19) has produced a bathymetric map showing contours at 10 m intervals.

METHODS Two bathymetric maps are presented. Figure 1 covers the are? between Port Nolloth and the Orange River and was compiled from ^'.A.N. Fair Chart No.16. Soundings on the fair chart are in fathoms and the original contours were drawn using these values, subsequently, the contours were converted to metres and, where 66 necessary, additional contours were interpolated to obt^'n a bathymetric map with a 2 m inter"al. As a result of the conversion, a certain amount of smoothing out or "averaging" became necessary, but at the scale drawn (1:100 000) this did not affec"; the accuracy to any extent. The bathymetric map of the area between the Orange River and Wreck Point (Fig.2), is derived from LWO sources. The outer soundings were obtained from echosounder records recovered during Cruise TBD 405 (Du Plessis 19801, whereas the inner soundings were irom EDO-Western 3,5 kHz seism;-- .^ccL'ds obtained during a small-boat cruise SAD 4/81. In deter-.ir i ng depths, the tidal variation was not taken into consideration since the range is in the order of "2 to 3 ft with spring tide 6-8 ft" (Hallair. 1964) which is with;:, the limits of accuracy of the data. The inner soundings were obtained from lines run 100 m or 200 m apart, with navigational fixes every two minutes. The outer soundings were taken from lines run at least 200 m apart, and fixes were taken every five minutes. The scale at which the data was plotted also varied considerably, with the inner values being at 1:2000 scale and the outer values at 1:25000 scale. As a result, the density of the data used to produce the bathymetric map differed considerably. The inner soundings ranged between 65 and 125 data points per km2/ with an average of 100 per km2, as opposed to the outer soundings which were between 35 and 50 per km2 (average 40 points per km2). In comparison, the average data density of the fair chart was 10 points per km2. Because of the differences mentioned above the two data sets were separated by a continuous line since the contours did not match exactly along the common boundary. However, general trends present in the topography are continuous across the boundary.

DESCRIPTION Relationship between bathymetry and geology:

Rogers (1977, p-l0) ha-1, de'-cr i hPd the innr-r shelf in this 67 area and notes that a close relationship exists between the onland geology and the bathymetric "style" of the inner shelf. South of Port Nolloth and northward as far as Cliff Point, feldspathic quartzites, arkoses and minor volcanics of the Sterkfontein Formation occur along the coast (SACS 1980). The bathymetry reflects this resistant bedrock in a narrow inner shelf, which has no clear shelf break, and slopes smoothly down to 75 m (Figs.1; 3a-6) where it dips beneath the onlapping mudbelt. These indurated recks grade into the "well foliated, iso-clinally folded" (SACS 1980, p.442) Hoiyat Suite consisting of quartz-sericite schists and quartz-feldspar-mica gneiss. Being softer, these rocks have not withstood coastal erosion as well, and consequently the inner shelf widens north of Port Nolloth (Fig.l) to a maximum of about 5 km. Several north-south oriented ridges extend across the inner shelf reflecting the irregular nature of the lithology. At Cape Voltas, the Holgat Suite lies with a thrusted contact against the Grootderm Suite (SACS 1980). The latter consists of "fissile chlorite-schists with intercalations of amphibole-epidote rocks and porphyritic lava" (SACS 1980, p.442). The Oranjemund Suite follows conformably on the Grootderm Suite, extending up to the Orange River. Dolomites, phyllites, quartz-chlorite and quartz-sericite schists and make up the suite. The softer strata have been eroded so that the coastline recedes farther east to the north of Homewood Harbour (Fig.l).

The major bathymetric features: The Orange River Delta: Figure 1 shows the southern extent of the submerged delta. Its form can be seen from the westward deflections of the smooth isobaths west of Wreck Point, located about 25 km south of the Orange River mouth. North of the mouth the delta extends for 68 over 80 km (Hoyt e_t a_l_ 1969), giving it an overall length of over 100 km. Cross-sections drawn from the Orange River mouth and Cape Voltas (Fig.3a) show the convex outline of the delta, with a slope of 0,1° over the first 10 km, steepening to 0,4° as the toe ot tre delta is reached. Hoyt e_t ai£ (1969) estimate the width of the delta to be about 25 Km opposite the river mouth, with a thickness of more than 60 m. Peacock Bank is a prominent rocky bank protruding through the "deltaic" sediments, standing proud by about 10 m from a depth of 28 m. There seems to be no apparent physiographic connection between it and the coast, although O'Shea's (1971) isopach map shows a wedge extending south of Peacock Ban* where the sediment thins to 0 m. This suggests that the bank forms part of a ridge similar to otters further south, although very subdued as a result of partial burial by the Orange River delta. The southern edge of the delta abuts against the inner shelf between Homewood Harbour and Wreck Point, deepening from -20 m to -60 m in the same direction. This delta edge coincides with the northern extent of the broad inner shelf which, from a position opposite Wreck Point narrows rapidly until, at Homewood Harbour, the shelf merges with the coastline (Fig.l).

The inner shelf between Homewood Harbour and Port Nolloth: The bathymetry shows a rugged undulating shelf, bounded to the east by an irregular coastline and to the west by a straight, continuous shelf edge (Figs.l and 2). The nearshore sediment wedge slopes down from the shore to about -20 m at an angle of between 1,1° and 2,0° and then flattens out to form the major part of the inner shelf between -20 m and -40 m (Figs.3a-4 and 3a-5). Several ridges, striking due south, cut diagonally across the inner shelf, generally standing 10 m to 15 m above the 69 surroundings (Figs.1,2 and 3a-5). These ridges separate areas of smoother, gentler topography, identified from the bathymetry as sediment-filled embayments (Figs.l and 2). Figure 2 shows that the two embayments between Homewood Harbour and Wreck Point have "inlets" at between 30 m and 35 m depth, opening towards the southwest. Sediments in these embayments are believed to differ from the sediments constituting the delta A li.}e drawn offshore from Homewood Harbour, dividing the area roughly in half, would separate a sediment population derived mainly from the Orange River, from a different population existing southward of the line. The latter would be derived mainly from a northward longshore drift, some of the sedment being funnelled through the inlets to fill the embayments.

It is noteworthy that the 30 m isobath outlines the base of the ridges and extends to the shoreward edge of the embayments (Fig.l). The 40 m isobath is relatively straight and marks the edge of the inner shelf (Figs.l and 2). This means that statistically, a significant portion of the inner shelf falls within the 30 m to 40 m depth zone, suggesting a possible ~ea level stillstand at this depth. This corresponds to othfr areas where evidence for a previous lower sea level is found (Flemming 1976, Murray e_t a_l 1970). While the Holgat River (Fig.l) does not show an equivalent prominent submarine valley like th^t of the Orange River found beneath the deltaic sediments (Hoyt e_t al^ 1969, Murray et a_l 1970), there does seem to be a deflection of the isobaths between 40 m and 50 m depth. The deflection:; suggest r"'*» existence of a valley, but a continuation towards the river mouth is not clear from the bathymetry alone. O'Shea (1971, map 3) shows a thin wedge of sediment, less than 6 m thick, extending from the Holgat River mouth. 70

The inner-shelf edge: The principal geomorphic feature of the edge is that it is extensive, linear and relatively steep. Rogers' (1977) maps show that the edge extends from south of the Buffels River, with a slight curvature to the north west, to Wreck Point, located about 130 km farther north. Within the study area, the inner-shelf edge is defined as lying between the 40 m and 73 m isobaths, and has a gradient of between l,i° and 1,9° (Fig. 3a). The inner- shelf edge maintains its smooth aspect to a point just north of Wreck Point (Fig.l), a;ter which it is dissected by several shallow valleys (or inlets) running northeast-southwest (Fig.2).

The Orange River mudbelt: The gradient of the inner-shelf edge (1,1° to 1,9°) contrasts with the gradient farther offshore where the average slope is 0,25° (Fig.3a-3 to 3a-6). This smooth offshore region is underlain by a thick deposit of terrigenous mud, originating mainly from the Orange River (Rogers 1977), and to a large extent impervious to seismic pulses (O'Shea 1971, map 3). According to the latter, this "acoustical blanking layer represents a sediment with a high gas content and which would therefore tend to strongly reflect acoustic pulses ..." (O'Shea 1971, p.21). These Recent muds cover the contact, at about 110 m, between Precambrian basement rocks and the seaward dipping Mesozoic and Cenozoic rocks on the middle shelf (Hoyt et^ al^ 1969, Rogers 1977) .

CONCLUSION Bathymetry may assist one with the interpretation of other aspects of marine geology, such as seismic profiles, sediment distribution patterns and subbottom geology. Within the study area, the bathymetry reflects: 1) various lithologies present 71 along the shoreline; 2) a possible sea level stand at between 30 m and 40 m below sea level; 3) the possible presence or two distinct sedimentary provinces, and 4) sediment pathways and depo-centres on the inner shelf.

REFERENCES Ahmed,A.A. 1968. Geological and mineralogical studies of sediments from the South-West shelf. RTPhil. Thesis, University of London.

Birch,G.F. 1975. Sediments on the continental margin off the west coast of South Africa. Ph.D, Thesis, University of Cape Town.

Du Plessis,A. 1980. Cruise Report: T.B. Davie Cruise 405. Unpublished Rep, geol. Sury. S. Afr. 1980-0176.

Flemming,B.W. 1976. Rocky Bank - Evidence for a relict wave-cut platform. Ann. S. Afr. Mus. 71:33-48.

Fowler,J.A. 1976. The alluvial geology of the Lower Orange River and adjacent coastal deposits. R.PFix. Thesis, University of London.

Hallam,C.D. 1964. The geology of the coastal diamond deposits of southern Africa (1959). In: Haughton,S.H.(ed). Some ore deposits in South Africa 11:671-728. .

Hoyt,J.H., Oostdam,B.L. and Smith,D.D. 1969. Offshore sediments and valleys of the Orange River (South and South West Africa). Marine Geology 7:69-84.

Murray,L.G. 1969. Exploration and sampling methods employed in the offshore diamond industry. In: Jones,K.J.(ed). Mining and Petroleum Geology 2:71-94.

Murray,L.G., Joyrt,R.H., 0'Shea,D.O'C., Foster,R.W. and Kleinjan,L. 1970. The geological environment of some diamond deposits off the coast of South West Africa. In: Delany,F.M.(ed). The Geology of the East Atlantic Continental Margin. Rep. Inst, geol. Sci. 70(3):119-141.

0,Shea,D.0'C. 1971. An outline of the inshore submarine geology of Southern and"~S~outh West~~Á"f rica and Namaqualand. Unpublished MTSc. thesis, Geol. Dept., University of Cape Town.

Rogers,J. 1977. Sedimentation on the continental margin off the Orange River~and the "KamTb DesertT Unpublished Ph.D. Thesis, Geol. Dept., University of Cape Town.

S.A.C.S. 1980. Stratigraphy of ': ,uth Africa. Part I. (Comp. L.E.Kent). Handb. Geol. S'jrv. S. Afr. 8:1-2. Fig. 1 'uL _LÏ L 1 _ _ 1 I Fig. 2 SECTIONS ACROSS THE INNER SHELF BETWEEN ORANGE RIVER AND PORT NOUOTH

v—O OMAMQE RIVER

-78

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0 Fig. 3 72

X QUATERNARY SEDIMENTOLOGY OF BOT RIVER LAGOON by J. Rogers

ABSTRACT The Quaternary sediments around the margin of the Bot River's closed estuary have been drilled and a succession up to 52 m thick has been analysed sedimentologically. Four main units have been identified. A basal Unit 1 is a lOrn-thick, angular, quartzose coarse sandy gravel rich in fragments of rock and vein- quartz and is mainly encountered in the palaecvailey under the barrier. It is interpreted as a fluvial basal gravel deposited during the early stages of a Wiirm Ila/IIb transgression. Unit 2 is 15 to 30 m thick and ib consistently muddy (muddy sand to mud). It is characterised by benthic foraminifera typical of inner-shelf or estuarine environments and is interpreted as a back-barrier lagoonal sediment deposited during the latter stages of the Wúrm Ila/IIb transgression. Due to compaction and its subsequent cohesiveness Unit 2 survived re-excavation during the Wurm I ID regression to -130 m, when the mouth of the palaeo-Bot River lay south of , between the Hangklip Ridge and Betty's Bay Bank. Unit 3 is 5-15 m thick and is a quartzose, medium sand under the barrier. Farther inland it can be a sandy mud and the unit can be rich in organic matter. Unit 3 was laid down during the Flandrian transgression. Unit 4 is the 15 to 3d m thick calcareous, quartzose medium sand of the barrier environment that is found on the beach, on the dunes and also on the washover fans that are periodically fed by storm-surges breaching the foredunes. There is evidence of a post-Glacial (5,5 to 2 Ka B.P.) barrier slightly inshore of the present coast and of a Late Pleistocene (125 Ka B.P.) shoreline beside the Lamloch swamps. 73

INTRODUCTION The estuaries of South Africa are currently the focus of intensive interdisciplinary research, but the Bot River estuary is one of the few which have been studied geologically (Theron et al,1981). Day (1981) has edited and co-authored a book on the ecology of South African estuaries as a whole. Begg (1978) has described the Natal estuaries in detail, whereas Heydorn a.id Tinley (1980) nave synthesized existing knowledge on Cape estuaries. A series of biological studies on South African estuaries was initiated by Day (1950) who reviewed existing knowledge. The first detailed study of an individual estuary was written by Scott et al (1952) on the Klein River estuary, east of . The latest in the series is a study of the Bot River estuary by Koop et al (in press). A synopsis of all available information on the Bot River estuary has been prepared by Koop et al (in press) as part of the series on Cape estuaries being produced by the Estuarine and Coastal Research Unit of the National Research Institute of Oceanology (N.R.I.O.) of the Council for Scientific and Industrial Research (C.S.I.R.). The present study is part of the Geological Survey's investigation of the Bot River estuary, the aim of which is to obtain a greater understanding of the geological history of a typical S.W. Cape estuary. This report does not include the results of seismic profiling and drilling in the lagoon itself. Instead, the results of onshore drilling are presented along with details of both onshore and offshore topography and geology.

DEFINITIONS Day (1980, p.198) has defined an estuary as follows: "An estuary is a partially enclosed coastal bouy of water which *s either permanently or periodically open to the sea and within 74 which there is a measurable variation of salinity duo to the mixture of sea water with fresh water derived from land drainage". The estuary of the Bot River falls into Day's (1981, p.5) category of closed estuaries which he defines as follows: "These are estuaries which are temporarily closed by a sandbar across the sea mouth". The term "lagoon" is described by Day (1981/ p.6) as a useful but less rigorously defined word: "Essentially, a lagoon is an expanse of sheltered, tranquil water..". In this report the expression "Bot River lagoon" will be used for convenience in preference to the more correct but wordier "Bot River closed estuary".

METHODS Geology and drainage The catchment and drainage-system of the Bot River were traced from the published geological sheet on Worcester (3319C) and Caledon (3419A) compiled by De Villiers (1966) at a scale of l:12r' 000. Onshore geology was traced from the above map, the lithological subdivisions within the Table Mountain and Bokkeveld Groups being omitted. The map was modified on the left (east) bank of the lagoon, south of the Afdaks River inlet. Bokkeveld crop out along the southern shore of the Afdaks inlet and continue along the shore to the inlet north of the Sonesta resort (Fig.l). In addition the area of aeolian sand neai Hawston was restricted to the area south of the latter inlet, thus shifting the boundary between calcareous coastal aeolian ssnd and siliceous "drift" sand southwards to the Hawston - Sonesta road. The map was reduced to 1:250 000.

Topography Onshore topography around the lagoon was traced at 100 m 75 intervals from the nautical chart SAN 120 ( to ) at a scale of 1:150 000. The fair charts FC 107 (False Bay - Southern Part; 1:50 000), FC 121B (Duiker Point to Cape Hangklip - Outer Soundings; 1:75 000) and FC 100 (Cape Hangklip to Cape Agulhas - Inner Area; 1:75 000) were used to prepare detailed bathymetry at 5 m intervals to a depth of at least 130 m. These bathymetric maps were reduced to a common scale of 1:150 000 and added to the topographic map. In this way palaeo-drainage patterns were mapped below present sea level (Fig-2) . The finer details of the onshore topography were traced from orthophotos at a scale of 1:10 000 obtained from the Office of Surveys and Mapping. These tracings were then reduced to a scale of 1:40 000, re-traced and then finally reduced to 1:50 000 (Fig.3).

Drilling The published geological map (De villiers, 1966) reveals that the shores of the upper and middle reaches of the lagoon are composed of Bokkeveld shales. Cenozoic sediments are therefore restricted to the shores of the lower reaches. Unfortunately these areas are often either marshes, steep, loose, coastal dunes infested with a dense cover of exotic rooikrans (Acacia cyclops)(01Callaghan, in Koop, in press) or beaches exposed to powerful surf action. Two techniques were used. Firstly, a series of 101 holes was drilled by uncased augering for 1,5 m (the ler.^th of an individual bit) and then sampling the lowermost 1,5 m. This entailed removing each overlying bit and was a tedious and exhausting task for the crew. The drilling time was also excessive so that most of the holes were drilled to bedrock without removing each bit. These 99 holes (B0T-1 to B0T-8, BOT- 10 to B0T-98 and BOT-100 to BOT- 101)(Fig.4) are therefore most 76 useful for determining a) depth to bedrock and b) sediment thickness. In the shallower holes they give a fairly accurate idea of sediment type, but in the deeper holes the sediment information deteriorates with depth. An advantage of the auger was its ability to drill within the dunes in terrain that was inaccessible to the larger percussion drill. Seven cased holes {BOT-102 to BOT-108) (Fig.4) were drilled with a percussion drill operated by the Directorate of Water Affairs of the Department of Environmental Affairs. The first hole, BOT-102, was drilled using aim long,127 mm (5 inch)- diameter tube inside a 152 mm (6 inch) casing. On reaching a compact, viscous clay horizon penetration was halted. The hole was therefore redrilled as BOT-103 using first a 203 mm (8 inch) casing, then reverting to a 152 mm casing to penetrate successfully to bedrock. These methods were used to drill two holes (BOT-103 and BOT-104) on the right (west) bank of the lagoon near Die Keel (Fig.4). A third hole (BOT-105) was drilled beside the Sonesta resort's lagoon-side parking area (Fig.4). The 4-wheel-drive Mercedes Benz lorry used to tow the drill encountered major problems when an attempt was made to drill on the barrier beach itself. The only access to the beach is at the extreme southeast end of the beach at Hawston (Fig.4). Having reached the beach the lorry became bogged down and before rescue vehicles could reach it the tide rose and the waves undermined the seaward side of the lorry, causing it to tip over to one side. This vehicle was retrieved but later was written off and a tractor fitted with special tyres proved to be an extremely efficient replacement. Three holes (BOT-106 to BOT-108) were drilled between the Sonesta mouth and the edge of Rooisand inlet and the palaeo-channel was located in BOT-107 (Fig.4).

Sediment sampling

The cores from the percussion drill were cleaned, 77 demarcated, measured and briefly described. Samples were taken at either side of every lithological boundary. Within long units samples were taken at 1 m intervals. Plastic sample bottles of 600ml capacity were transported in plastic crates.

Sediment analysis Textural analysis of the 231 percussion-drilled samples and the 29 auger-drilled samples from BOT-9 and BOT-99 was conducted by standard procedures, namely dialysis, wet-sieving, pipetting and settling (Brink and Rogers, 1979; Rogers, 1980). Carbonate contents were determined with a Karbonat-Bombe (Muller and Gast er, 1971; Birch, 1979). Components of the sand and gravel fractions were identified under a binocular microscope and component proportions were estimated (Ingram, 1965). Sediment colours were assigned using a Munsell Soil Colour Chart.

BOT RIVER CATCHMENT The Bot River catchment (Fig.l) lies on the coast of the South West Cape between Cape Hangklip and Danger Point or, more precisely, between the coastal villages of to the west and Hawston to the east. The village of lies in the centre of the catchment on the main Bot River and the town of Caledon is situated to the east on the banks of a tributary. Heydorn and Tinley (1980) estimate the area of the catchment to be 813 km2. The major left-bank tributary is the Swart, which cuts through a ridge of Table Mountain Group (TMG) sandstone, southeast of Botrivier village (Fig.l). Most of its course lies in wheatlands and pastures on shales and beneath mountains of Table Mountain Group sandstones. The Hopies River drains into the left bank in the uppermost reaches of the lagoon, whereas the Afdaks River drains into the 78 lagoon's middle reaches and forms a major emba/ment on its left bank. The catchment of the Onrus River has been added to Figure 1 because it is apparent that it has captured the headwaters of the Afdaks River. This may explain why the Afdaks, today a short stream, occupies a najor embayment of the Bot River lagoon. The major right-bank tributary is the Jakkals River which drains the Houhoek Mountains near Botrivier village. The Lamloch stream flows mainly into the freshwater swamps that lie inland of semi-consolidated barrier dunes towards Kleinmond. However parts of its TMG-derived "black" water (Heydorn and Tinley, 1980) mingle with the saline Bokkeveld-derived turbid or "white" water of the Bot River lagoon in the northwestern corner of the rectangular Rooisand inlet. The Isaacs River flows directly into the "black" water lagoon at Kleinmond. Due to the existence of a shallow overflow channel,within the Recent barrier,draining the Rooisand inlet's southwest corner/ the Lamloch swamps are a crucial part of the Bot River lagoon. Koop (in press) shows that old maps refer to the Little Bot River (Klein Botrivier) leading to Kleinmond, indicating that the link towards Kleinmond was more obvious in the past.

BOT RIVER LAGOON Apart from the Kleinmond overflow, the Bot River lagoon is very similar to the Klein River lagoon, just east of Hermanus (Scott et al, 1952) (Fig.2). The biological similarities, in particular, are stressed by Koop et al (in press) in their paper on the Bot River lagoon. The rivers enter their upper reaches via sandy channels flanked by muddy levees beside floodplains. On entering the variably saline upper reaches of the lagoon, flocculation occurs and sediment is deposited rapidly, forming a subaerial delta visible on the Klein River orthophoto. Short cores taken after the unseasonal February 1981 flood of the Bot 79

River (WJ11 is,1981) showed that oxidized brown sediment overlay older reduced black sediment for over 2 km downstream from the leveed river channel. Willis (1981) has presented a bathymetric chart of the lagoon produced by the Development Corporation (Figs.2 and 3). The level of the lagoon is very variable. When the mouth is open the level oscillates around mean sea level according to the state of the tide. Normally, however, the estuary is closed and the level varies between 1,4 m and 2,7 m above mean sea level (Fromme, in Koop, in press). The bathymetry is described relative to mean sea level, but any depths mentioned are clearly minimum depths and, for example, the deepest areas (over 2,5 m) may be up to 5,2 m deep, depending on the time of the year. The floor of the lagoon is mainly below sea level, the deepest areas being the Afdaks River inlet (below -2,5 m) and an area north of Sonesta near the channel to the mouth (Fig.3). Willis (1982) has published data on the sand content of the lagoon sediments which show that the deeper parts are mantled by mud (>90% mud, Folk, 1954). However the nearshore sediment south of the Afdaks River inlet and the quarry on the right (west) bank, as well as the sediment near the barrier is composed of sand (<5% mud). This textural zoning is reflected in the bathymetry, sand occurring between the narrow beaches and a marked steepening near mean sea level to the almost flat lagoon floor, covered with mud. Diver observations reveal that the two major textural types grade into one another over horizontal distances of only a few metres. The lagoon floor is not completely flat but slopes gently towards the left (east) bank. This is probably due to scouring upstream of the Sonesta mouth. Whether this scouring is artificial or not is a moot point as it is possible, due to its overflow to the Lamloch swamps, that the 80

Bot River lagoon never opens naturally at Sonesta. In a study of the normal estuary of the Keurbooms River at ,Reddering (1981, 1982) found that the carbonate content of the estuarine sediments . -ceeded 30% only between the ebb- and flood-tide deltas at the mouth. Most of the estuarine sediments contained 20-30% carbonate but the content dropped rapidly in the middle reaches of the Keurbooms lagoon. A similar pattern can be observed in the Bot River lagoon merely from analyses of core-tops of the six percussion boreholes. The beach material ranges from 18,8 to 21,9% and averages 20,0%, whereas at Die Keel, 1 km inland,the values have dropped to 5,7% at BOT-103 and to negligible amounts at BOT-104, a few hundred metres upstream. Therefore, within the sand above the mean-sea-level isobath, there is a strong seaward increase in carbonate content. This points to the coastal barrier as a source of carbonate-rich sand, which is brought into the lagoon, in the case of the Bot, chiefly by wind and by spring-tide washover. In terms of borehole interpretation it is vital to note that carbonate-rich, virtually mud-free sand characterizes the barrier environment.

BEDROCK TOPOGRAPHY The geological map of the area (De Villiers, 1966) was modified to restrict aeolian coastal sand to southwest of the Hawston-Sonesta road and the track to the inlet north of Sonesta (Fig.l). The BokkeveJd outcrop was extended along a line of low cliffs, up to 5m high, between the Lake Marina Yacht Club (Fig.4) on the left bank of the Afdaks River inlet, southwards to the inlet north of Sonesta. Inland of the aeolian coastal sand, the onshore topography shown in Figure 3 closely expresses bedrock topography. South-east of the Yacht Club at elevations of up to 30 m above sea level one finds iron-stained, well- rounded pebbles and cobbles of TMG sandstone resting on a gently 81 sloping surface of Bokkeveld . The scattered auger results show that bedrock usually only lies below sea level under the seaward half of the coastal duns between Sonesta and Hawston (Fig.4). A distinct palaeo-valley at least 51,9 m below sea level was located in BOT-107 between BOT-106 at the Sonesta mouth (-13,35 m) and BOT-108 (-15,74 m) due south of Die Keel. Bedrock lies well below sea level on the beach at Die Keel in BOT-103 (-39,8 m) rising rapidly to -24,8 m only tens of metres away in BOT-8 beside the vlei on the inland side of the dunes beside the beacn. However in BOT-104, a few hundred metres upstream, bedrock is a mere 3,1 m below sea level. The offshore bathymetry (Fig.2) shows that the nearshore sediment wedge smooths the isobaths to a depth of about 35 m. Irregular topography farther seaward indicates that bedrock crops out on the sea floor. Birch (1979) confirms this by finding sediment thicknesses of up to 10 m confined to a narrow 5 km-wide zone near the coast, on the basis of two seismic traverses between the 100 m isobath and the mouth of the Bot River lagoon. Flint (1971) estimated, from ice-volume calculations, that sea level may have dropped to about -130 m during the Wúrm lib glacial period, 18 *o 20 thousand years ago. Detailed bathymetry may therefore show what course the Bot River took during that period (Fig. 2). Assuming that the contours below 35 m reflect; bedrock topography, it is clear that the Bot River reached the lowered coastline, southwest of Cape Hangklip via a palaeo-valley between the Hangklip Ridge (Glass and Du Plessis, 1976) and Betty's Bay Bank (Fig.2). The palaeo-valley disappears up-valley beneath the sediment wedge opposite the palaeo-valley proved by drilling on the beach (Fig.4). 82

QUATERNARY SEDIMENTS A distinction has already been drawn between calcareous lagoonal sand grading upstream, around the lagoon edges, to siliceous lagoonal sand. Geomorphologically a similar, and related, distinction is made between calcareous barrier sand in beach, dune and washover environments and older, relatively thin, siliceous sand (mapped as "drift" sand by De Villiers (1966)) inland of the barrier. The barrier sand has been shown by drilling on the beach (Figs.9-12) to persist to depths of up to 15 m below sea level. The larger vegetated Junes, east of Sonesta, reach heights of 27 m in areas where bedrock is less than 10 m below sea level. Barrier sand therefore varies in thickness between approximately 15 and 30 m, thinning steadily seawards to bare bedrock within 5 km of the shore (Birch 1979) . In BOT-107 (Fig.11) Unit 4(S) the calcareous upper 14 m (3,1 to - 10,9 m) has the following characteristics: Very pale brown,

quartzose, shell-rich (15,12% CaC03), well-sorted (0,36 phi), subrounded to rounded, medium sand (1,18 phi) with traces of echinoid spines, rock fragments and benthic foraminifera. Unit 4(S) is underlain by Unit 3(S), 12,44 m (-10,9 to - 23,34 m) of yellow, quartzose, vein-quartz-bearing, rock- fragment-bearing, moderately well sorted (0,56 phi), very angular to well rojnded, slightly gravelly, slightly muddy, medium sand (1,41 phi), which is occasionally iron-stained. Similar sediments rest on bedrock beneath a barrier-sand layer in boreholes BOT-108 (Fig.11), in BOT-105 beside Sonesta and in BOT- 104 (Fig.8), near Die Keel. (BOT-103 (Fig.7) at Die Keel will be compared separately to BOT-107 over the palaeochannel.) Unit 3(sM) in BOT-107, is 1,06 m thick (-23,34 to -24,4 m) and is olive-gray, quartzose, plant-fragment-bearing, vein-quartz- bearing, moderately sorted (0,84 phi), very angular to well rounded, medium (1,79 phi) sandy mud containing some iron 83 concretions. Unit 2(M) in BOT-107 consists of 3,07 m (-24,4 to -27,47 m) of gray, quartzose, shell-rich, micromo11 use-rich, benthic-foraminifer-rich, fish-scale-bearing, plant-fragment- bearing, mud with traces of rock fragments and spicules and abundant diagenetic marcasite. Despite abundant calcareous material in the sand fraction (2,53%) the carbonate content is low (1,35%). Unit 2(sM) is similar and is composed of 4,93 m (-27,47 to - 32,4 m) of gray, quartzose, shell-rich, micromo11use-rich , benthic-foraminifer-rich (11,20% CaCC^}, sponge-spicule-rich, plant-fragment-bearing, moderately sorted (0,89 phi), very angular to well rounded, fine (2,34 phi) sandy mud with traces of echinoid spines and some diagenetic marcasite. Core was lost between -32,4 m and -41,9 m due to drilling problems but the driller reported the lithology as mud, which would increase the thickness of Unit 2(sM) from 4,93 to 14,43 m. The basal unit. Unit l(sG), consists of 10 m (-41,9 to -51,9 m) of light brownish gray, quartzose, vein-quartz-rich, rock-fragment-rich, well sorted (0,44 phi) very angular to subangular, coarse (0,28 phi) sandy gravel. Borehole BOT-103 (Fig.7), at Die Keel, 1 km inland from the modern beach, is similar overall to BOT-107 but will be compared in detail. Unit 4(S) is recognised in the upper 7,0 m (2,2 to -5,8 m)

as gray, quartzose, shell-rich (5,0% CaC03) , very well sorted (0,34 phi), subangular to rounded, slightly muddy, medium sand (1,45 phi) with traces of benthic foraminifera, microgastropods, echinoid spines and rock fragments. Unit 3(S) is similar in BOT-103 to its equivalent in BOT-107 in being carbonate-free, but in contrast it is organic-rich. It consists of 8,5 m (-5,8 m to -. m) of black, quartzose, vein- 84 quartz-bearing, well-sorted (0,50 phi), angular to rounded, slightlv muddy (peaty) medium sand (1,34 phi). Unit 3(sM),quartzose sandy mud, is not recognised in BOT- 103. Unit 2 is present in BOT-103 but in four subdivisions instead of two as in BOT-107. Unit 2(S) is 11,92 m thick (-13,2 to -25,22 m) and consists of dark grayish brown, guartzose, shell-rich, (39,3% CaCO^), benthic-foraminifer-bearing, echinoid-spine-bearing, moderately well sorted (0,50 phi), angular to rounded, siightly muddy medium sand (1,86 phi). Un^t 2(sM) is only 0,26 m thic'. (-25,22 to -25,48 m) - It is a gray, quartzose, shell-rich (48,40% CaCC^), benthic- foraminifer-bearing, ostracod-bearing, moderauely well sorted (0,60 phi), angular to rounded, fine (2,54 phi) sandy mud. Unit 2(mS) is 2,46 m thick (-25,48 to -27,94 m) and is olive-gray, quartzose, shell-rich (35,46% CaCC>3) , benthic- foraminifer-bearing , ostracod-bearing, plant-fragment-bearing, sponge-spicule-bearing, well sorted (0,45 phi), angular to rounded, muddy fine sand (2,56 phi). Unit 2(M) was recovered between -27,94 and -35,94 m, but core loss to -39,8 m in mud gives a total thickness of 11,86 m, although only 8,0 m was actually retrieved. The sediment is olive gray, quartzose, shell-rich (27,22% CaCO-j) mud. No equivalent of the basal unit 1 was found in BOT 103. The auger borehole, BOT-9 (Fig.5), which was drilled near the Sonesta resort workshop, and sampled in 1,5 m units can be divided into the following units: Unit 4(S) 19,5 m thick (11 to -8,5 m) Unit 3(sM) 1,5 m thick (-8,5 to -10,0 m) Unit 2(mS) 3,0 m thick (-10,0 to -13,0m) Unit 2(sM) 5,3 m thick (-13,0 to -18,3 m) 85

The second, similarly drilled auger borehole, BOT-99 (Fig.6) was drilled at the head of the Afdaks River inlet at the northern tip of the beach, between the river cnannel and a low barrier dune (Fig.4.) The beach is free of carbonate so falls into Unit 3(S) although the barrier a few metres away is probably Unit 4(S). The succession is as follows: Unit 3(S) 4,5 m thick (2,2 to -2,3 m) Unit 2(mS) 4,5 m thick (-2,3 to -6,8 m) Unit l(mS) 4,5 m thick (-6,8 to -11,2 m) Unit 1'sM) 4,5 m thick (-11,3 to -15,8 m) Although the data from this borehole are inferior to those from the cased percussion boreholes, the sedimentological details will be summarized to aid interpretation of boreholes drilled from a raft in the middle reaches of the lagoon, north west of BOT-99. Bedrock crops out along other parts of the lagoon shore and B0T-99*s site was inaccessible to the percussion drill. Unit 3(S) is pale yellowish brown, quartzose, well-sorted (0,43 phi), subangular to rounded, slightly muddy medium sand (1,90 phi). Unit 2(mS) is moderate yellowish brown, quartzose, microgastropod-bearing, shell-bearing, plant-fragment-bearing, rock-fragment-bearing, moderately well sorted (0,62 phi), angular to subangular, muddy f ine sand (2,27 phi) with traces of benthic foraminifera and sponge spicules. Unit l(mS) is pale yellowish brown, quartzose, vein-quartz-rich, plant-fragment-bearing, moderately sorted (0,76 phi) , angular to subangular, slightly gravelly, muddy medium sand (1,70 phi) with traces of microgastropods und shell fragments. This unit is regarded as basal due to the abundance of vein- quartz and the traces of carbonate components are ascribed tc contamination in an uncased borehole. Unit l(sM) is moderate yellowish brown, quartzose, plant- 86 fragment-bearing, moderately sorted (0,81 phi), subangular to subrounf.ed, slightly gravelly, medium sandy (1,89 phi) mud with traces of vein quartz.

DISCUSSION Tertiary history The Bot River lagoon is situated in a valley underlain chiefly by deeply weathered Bokkeveld Group shales, that are flanked across SW-NE-trending faults by mountains of Table Mountain Group sandstones (Fig.l). During the Tertiary period, following evidence of marine transgressions in the Saldanha are£i (Rogers, 1980; Siesser and Dingle, 1981), the Bot River valley was invaded by the sea up to the 90 to 100 m contour i.e. about 20 km up the present valley to beyond the village of Botiivier. Most evidence of these transgressions appears to have been removed by erosion. However a raised boulder beach, 30 m above sea level at Kogel Bay on the eastern shore of False Bay,n prominent terrace to 30 m at Kleinmond (Fig.3) and Hermanus (Krige, 1927; Taljaard, 1949, photos 14-16) seem to have counterparts beside the Bot River lagoon. Well-rounded, iron- stained pebbles and cobbles of Table Mountain Group sandstone are strewn over Bokkeveld Group shales at elevations of 25 to 30 m southeast of the Afdaks River inlet on the left bank and also northwest of this inlet on the right bank (Fig.3). In the absence of fossil evidence, dating is impossible, but a date prior to the Late Pleistocene (125 000 yrs Before Present (B.P.)) is probable. During the Oligocene a major sea-level regression to as much as 500 m below sea level (Siesser and Dingle, 1981) would have caused extensive erosion of the valley to well below present sea level. 87

Pleistocene history Shackleton and Opdyke (1973) produced evidence of a Late Pleistocene (Eemian) sea-level, about 6 m above present sea level, 125 000 years B.P. East of Kleinmond, the Lamloch swamF ' are protected from the sea by a densely vegetated (Acacia cyclops)(O'Callaghan, in Koop, in press) barrier dune which is up to 17 m high. On its seaward side it has beea eroded by wind action exposing calcretized rootlets (pedotubules), a characteristic feature of Late Pleistocene calcarenites nearby, e.g. at Hawston and at Die Kelders. It is suggested that this semi-consolidated barrier beside the Lamloch swamps represents the Late Pleistocene shoreline during the Eemian interglacial. Flint (1971) postulated a 130 m lowering of sea level during the Wiirir. lib glacial. This would have caused the Bot River valley to extend southwestwards beyond Cape Hangklip south of False Bay for a distance of 35 to 40 km from the present mouth (Fig.2). The palaeo-valley is best defined near the palaeo- mouth, where it lies between the Hangklip Ridge (Glass and Du Plessis, 1976) and the Betty's Bay Bank (Fig.2). The seaward flank of this bank is linear and parallels the NW-SE-trending isobaths of the middle shelf, probably due to faulting, but it is noteworthy that it intersects the middle shelf along the 130 m isobath. Faulted contacts between positive topography of Table Mountain Group sandstone and negative topography of Bokkeveld Group shale has been pinpointed onshore by De Villiers (1966) as well as offshore in this region by Gentle (1971) and by Glass and Du Plessis (1976). Betty's Bay Bank is therefore probably composed of Table Mountain Group sandstone. However, Gentle's map (1971, Fig .70-II1-4) , based on widely-spaced side-scan-sonar traverses and isolated dredge samples bears the interpretation of Bokkeveld Group bedrock with a NE-SW-trending fault. A second phase of detailed side-scan-sonar coverage followed by sampling 88 and underwater photography would resolve the inner-shelf geology of the Bot River's palaeo-valley. Wright (1964), Murray et al (1971) and Plemming (1976) have observed an abrasion surface on the inner shelf which has been attributed by Tankard (1976) to a Late Pleistocene interstadial sea level of -20 m dating to between 47 and 25 Ka B.p. The microfossiliferous muddy sediments of Unit 2 in the palaeo-valley beneath BOT-107 lie between -24,4 and -41,9 m. The bentnic foraminifera are long-ranging extant species typical of either lagoonal or inner-shelf environments (R.A. Martin, personal communication) and are not age-diagnostic. It is possible that the palaeo-valley was scoured of earlier sediments during the post-Eemian, Wiirm Ila regression. Unit l(sG) was then deposited as base level rose during the Wiïrm Ila/IIb interstadial as a fluvial basal gravel. Unit 2(M and sM) were probably deposited as back-barrier muddy sediments in a sheltered lagoonal environment. The unit is 17,5 m thick in BOT-107 and post- depositional compaction would have caused considerable loss of interstitial water and increased the cohesiveness of the unit. Consequently when sea level dropped to -130 m during the Wúrm lib glacial period, 18 to 20 Ka B.P., the rejuvenated Bot River was not able to erode Unit 2 completely.

Holocene history During the Flandrian transgression any barrier formed at the mouth of the palaeo-Bot River would have been in a microtidal high-energy west-coast-swell environment (Davies, 1964). Estuaries tend to be closed and the lack of tidal inlets causes an abundance of washover fans as storm surges overwhelm the barrier dunes and wash sediment over to form wide back-barrier flats of calcareous sand on the seaward margin of back-barrier lagoons. Onshore winds, particularly in summer, deflate the 89 wider, finer-grained beaches that build up when wave energy abates. Washover fans, back-barrier flats and unusual, coast- perpendicular, sinuous dunes all characterise the modern Bot River lagoon's seaward edge, particularly in the Rooisand-Sonesta area. However washover fans also break through the semi- corsolidated barrier between Rooisand and Kleinmond and deposit calcareous sand beside the Lamloch swamps (Fig.3). The processes observed in the modern phase of the Bot River lagoon can be expected to have operated during the Flandrian transgression, wind and wave energy probably being somewhat higher at the start of the transgression due to slight equatorward shifts of the climatic celts (Tankard and Rogers, 1978). This process of shoreface retreat during periods of rising sea level was postulated by Bruun (1962, in Elliott, 1978). The apparent dearth of sediment below about -50 m (Fig.2) indicates that this landward "sweeping" of sediment was very efficient along the path of the palaeo-valley. During the Flandrian transgression the base-level of the palaeo-Bot River would have been raised steadily so that the angular, non-calcareous, fluvial sand and mud of Unit 3 was deposited. Marshy environments on the margin of the river or the developing lagoon would cause peaty sediments to be deposited as at Die Keel, where the lateral equivalents of peaty sediments in a local vlei were drilled beneath a thin layer of barrier sand beside the lagoonal beach (Fig.7). At Lagoon, Flemming (L977) obtained evidence for a post-Glacial high sea level between 5500 and 2000 yea».s B.P. to a height of about 3 m above present sea level. Deflation of coastal dunes about 100 m inland of the present beach on the northwestern edge of Hawston (Fig.3) revealed a deposit of well- rounded, now sand-blasted, discoidal pebbles of Bokkeveld Group rocks. This probably represents a beach formed during a post- 90

Glacial high sea-level, the coast having subsequently prograded to its present position. It is suggested that the post-Glacial coastline may have extended from Hawston through the Sonesta resort, across the lower reaches of the modern lagoon to the vegetated dune leading to Die Keel and the barrier dunes fringing the landward side of the Rooisand inlet. The coastline then merged with the seaward edge of the semi-consolidated dunes towards Kleinmond. Evidence of progradation east of Kleinmond is the development of low, grass-stabilized, beach ridges between the beach and the wind-eroded strip along the seaward edge of the semi-consolidated barrier dune. Progradation was only encountered by Heydorn and Tinley (1980, END MAP) at five localities during an aerial survey of the Cape coastline, viz. (2), south of Yzerfontein, east of the Breede River mouth, and at Kleinmond. The calcareous sand of Unit 4 therefore has been frequently recycled through the various subenvironments of the barrier facies during the Flandrian transgression. Fromme (Fig.4 in Koop, in press) has mapped the artificial breaching of the barrier which occurred on 11th August 1981 near the north western end of the Rooisand inlet. He also mapped a small seaward-facing escarpment which the outflowing lagoon water oarved from peat beds rich in root material. Before the breaching the peat, an extension of the Lamloch swamps was overlain by calcareous sand in a washover fan that had smothered the original reedbeds. The channel through the barrier provided an excellent section of the back-barrier peat, the smothering washover fan, the foredunes, the backshore, the berm and the foreshore. A detailed photographic record is housed in the regional office of the Geological Survey in Bellville. The barrier was subsequently artificially breached near Sonesta on 10th October 1981. Photographs of the breaching along with photographs taken a week later of the fully developed, tidal 91

inlet are also housed in Bellville. Fromme (in Koop, in press) reports that the Rooisand opening stayed open for 3 months in comparison to the Sonesta opening which was maintained for only 2 months. He attributed the rapid closing firstly to opening after the rainy winter season and secondly to powerful longshore drift from Hawston towards Kleinmond. He also reported northwestward movement of turbid water near inlets as well as NW-orientated ebb-tidal shoals. Fromme (Fig.6, in Koop, in press) noted that 52,8% of deep-sea waves were southeast of a line drawn perpendicular to the coast and this suggested northwestward longshore drift. The bathymetric map (Fig.2) shows that the isobaths are broadly parallel to the average wave front as well as to middle-shelf isobaths and the shelf break. Relatively little refraction is therefore suggested. However the greater tendency for the wave fronts to attack from the south-eastern end of the spectrum seems to sweep sediment towards the inner shelf off Kleinmond. The smoother isobaths above -50 m indicate an accumulation of sediment, east of Betty's Bay, that was first depicted by Glass and Du Plessis (1976).

CONCLUSIONS A geological investigation of the Bot River lagoon has revealed a complex Quaternary history. The sediments drilled beneath the modern barrier carry evidence of two regressive- transgressive episodes. Fluvial quartzose sandy gravel in a palaeo-valley is overlain by cohesive, microfossiliferous back- barrier mud, overlain in turn by fluvial quartzose sand, capped by modern calcareous barrier sand. The microfossils are extant and suggest a Quaternary age for the mud. The palaeo-valley is traceable on a detailed bathymetric map (Fig.2) to a palaeo-mouth at -130 m, south of False Bay. It was probably cut originally during an Oligocene regression to 92

approximately -500 m (Siesser and Dingle, 1981) and then periodically filled and re-excavated during subsequent sea-level oscillations. The observed sequence in the palaeo-valley probably reflects re-excavation during a Wurm Ila regression, followed by deposition of fluvial gravel and then back-barrier mud during a Wiïrm Ila/IIb interstadial (47 to 25 Ka B.P.). The cohesiveness of the compacted mud then allowed it to survive until it was overlain by fluvial sand and, finally, barrier sand. Retreat of sea level from a post-Glacial high of 2-3 m (5,5 to 2 Ka) has caused some progradation of the coastline. An unusual interaction between the major, saline Bot River lagoon and the minor, freshwater Lamloch swamps towards Kleinmond has resulted in the complex but instructive system we see today.

ACKNOWLEDGEMENTS This report complements the Geological Survey's detailed geophysical study of the sediments beneath the lagoon itself, as well as drilling from a raft between the rocky shores of the middle reaches of the lagoon. The work was performed while the author was a member of the Geological Survey, based at Bellville, and the study was completed at the University of Cape Town. The author acknowledges the support of the Director of the Geological Survey, the guidance during the survey of Dr J N Theron and Mr A Du Plessis and the assistance of Mr R Woollatt in organising the auger-drilling team as well as determining the collar elevations of the augered boreholes. The Directorate of Water Affairs is thanked for the invaluable percussion drilling in difficult conditions. They suffered considerable inconvenience when the rig was stranded during an unseasonable flod in February 1981 followed by the loss of a lorry on the beach. The drilling inspector Mr Maritz and the driller himself, Mr Kunnecke, are to be praised for overcoming the above obstacles. Thanks are also

r> 93 due to Dr H Swart and Mr J Zacks of N.R.I.O. for providing collar elevations of the percussion boreholes. Dr A Heydom and Mr I Bickerton of N.R.I.O's Estuarine and Coastal Research Unit kindly provided me with a draft of Mr Koop's report on the Bot River lagoon and have consistently supported this geological study. Finally I should like to pay tribute to the late Mr A J Morris, an officer of the Cape Provincial Department of Nature Conservation who enthusiastically encouraged scientific research at Bot River Lagoon and who introduced us to the complexities of the conflicting interests inherent in the use of this attractive area.

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Day,j.H. 1980. What is an estuary? S^ Afr. J. Sci. 76:198. Day,j.H.(ed) 1981. Estuarine ecology with particular reference to southern Africa. Cape Town: Balkema 1-411. 94

De Villiers,J. 1966. Geological map of 3319C-Worcester and 3419A -Caledon. Map Dept Mineral and Energy Affairs geol. Surv. *L A£r» Elliott,T. 1978. Clastic shorelines. In: Reading,H.G. ed. Sedimentary environments and facies. Oxford: Blackwell. Flemming,B.W. 1976. Rocky Bank - evidence for a relict wave-cut platform. Ann. S. Afr. Mas. 71:3_i-48. Flemming,B.W. 1977. Langebaan lagoon: a mixed carbonate-silic- iclastic tidal environment in a semi-arid climate. Sediment. 18:61-95. Flint,R.F. 1971. Glacial and Quaternary geology. New York : Wiley. Folk,R.L. 1954. The distinction bwtween grain size and mineral composition in sedimentary rock nomenclature. Jj_ Geol. 62:344-365. Fromme,G.A.W. (In press). Estuary characteristics; Mouth dynamics. In: Koop,K.(compiler}. Estuaries of the Cape. Part II: Synopses of available information on individual systems. Bot/Kleinmond system (CSW 13)(Rep.No.18). Res. Rep. S. Afr. coun. sci. ind. Res. 417:9-19. Gentle,R.I. 1971. Pre-Quaternary geology of th-> continental shelf between Cape Infanta and Cape Town. Tech. Rep. S. Afr. nat. comm. oceanogr. Res. mar. Geol. Progm. 3:13-27. Glass,j.G.K. and Du Plessis,A. 1976. The bathymetry of False Bay as an indicator of sea floor geology. Proc. 1st interdisciplinary Conf. mar, freshwater ResT S. Afr• Fiche 6 D5-E6. Heydorn,A.E.F. and Tinley,K.L. 1980. Estuaries of the Cape. Part I. Synopsis of the Cape coast. Natural features, dynamics and utilization. Res. Rep. S. Afr. Counc. sci. ind. Res. 380:1-96. Ingram,R.C. 1965. Facies maps based on megascopic examination of modern sediments. J^ sedim. Petrol. 35:619-625. Koop,K. (In press)(compiler). Bot/Kleinmond system (CSW 13). (Rep.No.18). In: Heydorn,A.E.F. and Grindley,J.R. eds. Estuaries of the Cape. Part II. Synopses of available information on individual systems. Res. Rep. S. Afr. Coun. sci. ind. Res. 417:1-58. Koop,K., Bally,R. and McQuaid,C.D. (In press). The ecology of South African estuaries. Part XII: The Bot River, a closed estuary in the south . S. Afr. J. Zool. 18:1- 14. Krige,A.V. 1927. An examination of the Tertiary and Quaternary changes of sea-level in south Africa, with special stress on the evidence in favour of a recent world-wide sinking of ocean-level. Ann. Univ. Stell. 5(A1):1-81. 95

MQller,G and Gastner,M. 1971. The "Karbonat-Bombe", a simple device for the determination of the carbonate content in sediments, soils and other materials. Neues jb. Miner. H10:466-469. Murray,L.G., Joynt,R.H., O'Shea,D.O'C., Foster,R.W. and Kleinjan,L. 1971. The geological environment of some diamond deposits off the coast of South West Africa. In: Delany,F.M. ed. The geology of the East Atlantic continental margin. Rep Inst, geol. Sci. 70(13):119-141. 0'Callaghan,M. (In press). Flora. In: Koop,K. (compiler). Estuaries of the Cape. Part II: Synopses of available information on individual systems. Bot/Kleirimond system ;CSW 13).(Rep.No.18). Res. Rep. S. Afr. Coun. sci. ind. Res. 417:24-28. ~ Reddering,J.S.V. 1981. The sedimentology of the Keurboms estuary. Unpublished M.Sc. thesis, Geology Department, University of Port Elizabeth. Reddering,J.S.V. 1982. The sedimentary ecology of the south- estuaries. Abst. 3rd Symp. sedim. Div. geol. soc. S. Afr. 50-52. Rogers,J. 1980. First report on the Cenozoic sediments between Cape Town and Elands Bay. Rep, geol. Surv. S. Afr. 1980- 165:1-64. Scott,K.M.F., Harrison,A.D. and Macnae,W. 1952. The ecology of South African estuaries. Part II. The Klein River estuary, Hermanus, Cape. Trans. R. Soc. S. Afr. 33:283-331. Shackleton,N.J. and Opdyke,N.D. 1973. Oxygen isotope and palaeo- magnetic stratigraphy of Equatorial Pacific core V28-238: oxygen isotope temperatures and ice volumes on a 10^ year and 106 year scale. Quatern. Res. 3:39-55.

Siesser,W.G. and Dingle,R.V. 1981. Tertiary sea-level movements around southern Africa. J. Geol. 89:83-96. Taljaard,M.S. 1949. A glimpse of South Africa. Stellenbosch: The University Publishers and-Booksellers. 1^226. Tankard,A.J. 1976. Cenozoic sea level changes: a discussion. Ann. S. Afr. Mus. 71:1-17. Tankard,A.j. and Rogers,J. 1978. Late Cenozoic palaeoenvironments on the west coast of southern Africa. J^_ Biogeogr. 5:319- 337. Theron,J.N., Du Plessis,A. and Rogers,J. 1981. Depositional history of the Bot River estuary. In: Heydorn,A.E.F. Proceedings of workshop on research in Cape estuaries. Rep. S.*Af r • Cou"» sci. ind. Res. T/SEA 8111:109-113. Willis,J.P. 1981. Geochemistry and size analysis of sediments from estuaries in the southern Cape. In: Ueyiorn,A.E.F. (compiler). Summary and background notes: Worksession on the Bot/Kleinmond system. Univ. Cape Town, 21 September, 1981- (Unpublished) 1-2S. 96

Willis,J.P. 1982. Geochemistry and physical characteristics of sediments from Botriviervlei. Progr. Rep. S. Afr. nat. Comm. oceanogr. Res. Wright,J.A. 1964. Gully pattern and development in wave-cut oedrocK shelves north of the Orange River mouth. South West Africa. Trans, geol. Soc. S. Afr. 67:163-171.

Contours at 5 m intervals to 100 m from orthophotos. 1_ 1 2 3 4 5 km Figure 3 jir 27 -' ...t _. 1«"í BOT RIVER LAGOON BOREHOLE LOCATIONS AND BURIED BEDROCK TOPOGRAPHY

•75 O')*

KLEINMOND

4 30 Avr ,7. i. V. •* '3

• Cased, tubed percussion borehole • Uncased auger borehole (1,5m. intervals) • Uncased auger borehole fy> Deplh ol buried bedrock relative to sea level

SÍi^HAWSTON

1903Í llLiii. I Fiqure 4 BOT-9 Drilling Sand Coarse- Fraction Sand Sor»d Metres Method Roundness Components Ca COT Mean Size Mud vfS fS cS veS Sorting (%) (Phi, (%) (%) (%) (%) (7c) (Phi) Qz Sh 50 G-l 0 I 2 3 4 Mud 0 IQO 0 2,5 j5 0 25 0 .„5 ^ 12-j-O i • i i 1.1. • 1. •*. • »-i 10- -5 5- •10 Seo Level e -5 •20 -10- I * •25 * -15- -17,3 -^293 i

LEGEND Drilling Method Coarse-Fraction Components Sand Mean Size

• Tub* Dominant Qi Quarti . • l • • • • • < Sh Shell Fragments K'MVtÍ Coarse Sand Subordinate bF Benthic Foraminifera k\J Bit and Bailar PI Plant Fragments Medium Sand Minor VQ Vein Quarti Sand Roundness Ost Ostracodes Trace RF Rock Fragments • Rounded Fine Sand * Angular

Figure 5 BOT-99 Drilling Sand Coarse Fraction Sood Sand Vnires Method Roundness Component- Co CO5 Mean Size Mud vfS fS mS cS vcS Sorting (%) (phi) (%) (%) (%) (%> (%) '%) (p».0 Qz 0 50 6 "I 0 I 2 3 4 Mud 0 100 0 25 0 50 0 50 0 25 ( 25 0 0.5 I 3 Soa Levei- Sh bF PI I 10 i 15 : -15 Gravel (%)

Legend Fig. 5 Figure 6 T 103 Drilling Sand Qaarss- Fnxflvi Sand ftJ° - Metre Method Roundness Components Mtan Size Mud »ts fS IDS 9Pno cS »cS Sorting (Phi) (%) (%) (%) (%) (%) (%) (phi) 6-1 O I 2 3 4 Mud O 1000250 1000 1000 50 O 25 O Oft 2* T0 Sea Lev*

-5- O -10

-15 20 -20-

25 -25- -30 -30- -35 I -35- A •40 -39gB- -42,0

L45.0

legend Fig.5 Figure 7 BOT-104 Drilling Sc-.d Coorse- Fraction Sond Sond Me ties Metrod Roucdress Components •:•! co vfS

•20 -20 Bokkeveld Shale 25

Legend Fig. 5 Figure • r> o i o I «0

cL

U) o •5 5-

\ 4) 2 9J í

"O . . . . ."5 ::::::: :::> si:::: *-|:::: !'1::: : ::::::::: *Í:::: ;:::.:;; ::j^ n « >

-4 I»

h ÉS

19 ' C 0»»0<»M—•»''

t»1 i$

in I -i—' r—> 1 I • l—' ^—' I ... BOT-106 Dri'ling Son* Coorse- Fraefion Sand :>and Components CaCCK vfS fS mS c3 Metits Met nod Roundness Mean Size Mud vcS Sorting (%r ipni) (%) (%) (%) (%) (%) (phi) Qz Sh o 50 6-1 0 1 2 3 4 Mud 0 100 0 25 0 50 0 100 0 50 0 25 0 0,5 i 1 i • i » ', \ ,f • 1. • !_• lb. •«: 4 - 0 y: • l \ .. vei S ^••••••••••" \ fe«X<>>>.*J* 1 5 ^ T jT~ * • /M ._ i::::::::;::: ÍT -:0- m^ 15 |l 1 ^iiiiiii: -ii.25- ^* 6o Gravel II LI -I -15- •20 (%) Weathered Bokkeveld Shale -20- -25

-25-

Legend Fig. 5 Figure 10 BOT-107 L'rilling Sond Coarse-Fraction Sand Sand Metres MethcJ Roundness Components C0CO3 Meon Size Mud vfS fS JCS sorting (phi, v%, (%) (%) (%) (ph.. 50 ,r,-j 0 t 4 3 4. Mud 0 i&p Q dp Q «3 C 3,1 rO

-5

I*

3* Si -2: Í* SI i-jr

3CH h-5 i 1 -35- 4C Core Loss -iCH '45 A 45' • •••••••• i • ••••••••1 5t •-V.V.V.V.' -50- I • • • ».»^» •J ....»«.. 100 ,0 VMsorhered Grovel Bokkeveld Shale (%)

Legend Fig. 5 Figure 11 DrlRino. Sond Coorse-Froctlon Sand Sand •OT-10* CoC0 mS cS vcS Sorting Metres Method Aewdrast Component* 3 Meon Sue Mud vfS tS (%) (phi) (%) (%) (%) (%) (%) (%) (phi) Qz Sh 0 50 6-1 O I 2 3 4 MudO 100 0 25 0 25 3,7 TO I I 1 L, I I !

Sea Lev*! " _

10

-10' i -15

-lb -15,74- 20 0 „ ,100 Waotherrd Gravel (%) 25 BokKeveld Shale l27,0

legend Fig.5 Figure 12 97

XI BEACH MORPHODYNAMICS IN RELATIONSHIP TO WAVE ENERGY, GRAIN SIZE AND INTERNAL SEDIMENTARY STRUCTURE by B.W. Flemming

INTRODUCTION Sandy beaches have for a long time eluded a rational quantitative description, although it has been evident that they responded quite sensitively to changes in environmental conditions (Bascom, 1964; Hayes, 1972). Over the past ten years this situation has changed dramatically. Many systematic responses have been recorded in relationship to variations in wave energy and this has in more recent years led to a semi­ quantitative model of beach classification. In this contribution the recent literature on beaches is reviewed by discussing the physical parameters characterizing beach and nearshore environments, especially beach morphodynamics as a function of wave energy, grain size/beach slope relationships and variations in internal sedimentary structures along beach and nearshore profiles.

MORPHODYNAMIC BEACH CLASSIFICATION For some time now it has been known that beach profiles undergo periodic modifications in relationship to changes in the seasonal wave climate (e.g. Bascom, 1964; King, 197 2). However, until recently it was not possible to describe such qualitative observations in quantitative terms. The first three-dimensional model was developed by Sonu (1973), whereas Guza and Inman (1975) for the first time described the beach cycle in terms of the associated surfzone dynamics. They introduced the terms dissipative or high energy beaches and reflective or low energy beaches (cf. Fig.l). It was found that all beach systems 98 constantly adjust between more dissipative and more reflective states. This approach has recently been significantly expanded by Australian scientists, who have identified successive morphodynamic stages in the process of adjustment between the completely dissipative and totally reflective extremes (e.g. Short, 1978, 1979, 1981; Wright, 1981, 1982; Wright et al., 1978, 1979, 1982) . To distinguish between totally reflective and strongly dissipative wave types these authors have used the surf scaling or reflectivity parameter

e= a;w2/g tan2B where a; is the wave amplitude near the breakpoint, w = 2T/T where T is the wave period, g is the gravitational acceleration and B is the beach or inshore slope (cf. Guza and Bowen, 1975). For total reflectivity c must be <1, whereas highly dissipative waves have values of e>33 (cf. Wright et al., 1979). More recently this morphodynamic classification of beach

stage (Bs) has been reduced to a simple empirical relationship between breaker height (H^), wave period (T) and the settling

velocity of the mean grain size in m/sec (Ws) such that

Bs= Hb/WST

In this classification scheme reflective beaches characteristically score values <1, whereas dissipative beaches score >6 (Wright and Short, 1982). various intermediate beach stages can also be distinguished, whereby each stage can be recognized by typical morphological modulations of the beach and nearshore profile (Table 1). Thus, the completely dissipative beach system is characterized by multiple shore-parallel bars, a 99

wide and often rhythmic surf zone and a narrow beach, whereas a totally reflective beach is typically non-barred, has a narrow surf zone and a much wider beach. In the constructive phase of a beach cycle, i.e. starting with a dissipative system after a high-energy event, a beach adjusts towards a more reflective stage by progressive shoreward migration of the offshore bar systems, which will eventually be welded entirely to the beach, provided the duration of the low-energy conditions persist long enough. In many cases, for example along the Sou^h Australian and most of the South African shoreline, the totally reflective state is rarely achieved because of the relatively high average wave climate. Alternatively, many sheltered shorelines, that are occasionally exposed to high-energy events, will retain the dissipative beach profile almost unchanged as a relict feature for relatively long periods of time because the average energy conditions are so low that they are unable to modify the profile produced during the previous high-energy event. Examples of this are presented by Da-'idson-Arnott and Greenwood (1976) and Exon (1975) from the southern gulf of St. Lawrence and the Western Baltic respectively. Other recent studies dealing with beach morphodynamics have been published by Greenwood and Davidson- Arnott (1979), Chappell and Eliot (1979), Goldsmith et al. (1982) and Bowman and Goldsmith (1982). The morphodynamic model outlinea above will require further refinements, especially as examples have been described in the literature which, at first sight at least, do not appear to fit the model (e.g. Clifton et al., 1971; Hawley, 1982). A further quantification of beach processes has been suggested by Aubrey et al. (1982) who believe that successful predictions of weekly changes in beach profiles could be achieved if a sufficiently long record of local wave climate could be coupled with a longshore transport model.

fm 100

Table 1. Morphodynamic beach classification (after Wright and Short, 1982)

beach dynamic wave beach morphodynamic beach stage stage state energy profile

strongly low steep high berm with convex reflective (H<1 m) but smooth beach face welded bar often with ridge and runnel systems and beach cusps intermediate transverse bars, or anvil- shaped attached crescentic bars, low-tide terraces with irregularly spaced rip channels, incipient megacusps transverse bars with even­ ly spaced rips or un­ attached crescentic bars, megacusps. transverse to linear bars with widely spaced rips, cusps strongly high gentle single or multiple para- dissipative (H>3 m) but rhythmic llel bars, sometimes very widely spaced rips

GRAIN SIZE/BEACH SLOPE RELATIONSHIP

A systematic relationship between the mean grain size of a 101

beach and its slope as a function of wave energy was observed and quantified by Bascom (1951), Inman et al. (1963) and Wiegel (1964). In principle there is a negative correlation between the mean grain size measured in phi (D phi) and the beach slope measured in degrees (S) . Thus, for any given grain size a beach slope will always increase with decreasing wave energy (cf. Fig.2). This in fact explains why at a particular locality the low-energy, reflective beach profile is usually steeper than its high-energy, dissipative counterpart. Alternatively, at similar energy levels a coarser sand will always frrm a steeper slope than a finer sand. By recalculating and rearranging the data presented by Wiegal (1964) the author has been able to transform this relationship into a more convenient form which lends itself towards mathematical quantification in form of empirical equations (Table 2). Most beach sediments are relatively well size-sorted and major differences in grain size usually reflect corresponding differences in relative energy levels (cf. Bascom, 1951). Of course there are exceptions to this rule; for example, when a localized source supplies coarse material to a sheltered beach or when an exposed beach receives a surplus of fine sediment.

Table 2. Empirical formulae for estimating beach slope as a function of grain size and wave energy and vice versa (cf. Fig.2)

-2 16 7 0 463 Low Wave Energy : Dpni=3.17S ' or S = (3.1 /Dpni) ' Intermediate . _. „ .. -2 04 0 4 Wave Energy : Dpni=2.72S • or S = (2. 72/DpM) - 9

7 0 0 5 High Wave Energy : Dpni=2.25S" - or S = (2.25/Dphi) -

S = beach slope in degrees ; Dphi= mean grain size in phi 102

PROFILE GEOMETRY AND INTERNAL STRUCTURES From the above discussion it is evident that the identification of morphodynamic beach stages, together with their temporal and spatial variability, is an important prerequisite for the causal interpretation of beach profile geometry as a function of wave energy (cf. Hawley, 1982). On the other hand, internal sedimentary structures are the only means of interpreting fossil beach deposits. A discussion of the major structures observed in a variety of different morphodynamic beach systems may therefore provide valuable clues to the interpretation of the fossil record. Three examples have been chosen - a parallel-barred or fully dissipative beach stage (Fig.3A) an oblique-barred or intemediate beach stage (Fig.3B) and a non-barred or reflective beach stage (Fig.3C). In each case the observed facies zonation is illustrated by cross-sections which also show schematically the most characteristic internal structures recorded along the profile. The great variety of facies-specific structures is immediately apparent. This feature can be further enhanced by variations in sediment texture. In the pardlel-barred, dissipative case the sequence of distinct facies or macrostructures can be summarized as follows: a nearshore/offshore transitional facies is followed by a seaward outer-bar slope, an outer-bar crest, a landward outer-bar slope, an outer trough, a seaward inner-bar slope, an inner-bar crest, a landward inner-bar slope, an inner trough, a swash/trough transitional zone, a swash zone and finally a high berm. The oblique-barred case is in many respects similar, except that there is now a more pronounced rip-channel facies and a lateral rhythmic succession of repetitive cycles. The high-energy, non- barred situation, on the other hand, is very different. The 103 nearshore/offshore transitional facies is followed by an outer rough zone consisting of lunate megaripples, an outer planar zone, an inner rough zone, an inner planar or swash zone and eventually a high berm perched in the front of a wide beach. In addition there can be symmetrical and asymmetrical ripples in most of the zones, whereas small ste ding waves, antidunes and rhomboidal back-wash patterns commonly occur in the swash 2one. Furthermore, at the foot of the backwash there is usually a well- defined step at which the coarsest material is concentrated. This step migrates across the intertidal zone in the course of each tidal cycle. DISCUSSION AND CONCLUSIONS Beaches are highly dynamic environments which respond sensitively to short and longterm fluctuations in energy levels. All beach systems constantly adjust between more dissipative and more reflective states. The state of a beach observed at any particular point in time thus simply reflects its dynamic adjustment to the coincidenta1 energy conditions prevailing at that time. Exceptions can occur in very low-energy situations where the observed beach stage may represent a relict feature of a previous high-energy event. It is therefore evident that the identification of any particular morphodynamic beach stage must take its temporal and spatial variability into account if the beach system is to be fully understood. This is of particular importance in the establishment of sound coastal management policies.

REFERENCES Aubrey,D.G., Inman,D.L. and Winant,C.D. 1980. The statistical prediction of beach changes in southern California. J. Geophys. Res. 85:3264-3276.

Bascom,W.N. 1951. The relationship between sand size and beach face slope. Trans. Am. Geophys. Union 32:866-874. 104

Bascom,W.N. 1964. Waves and Beaches. Doubleday & Co., New York, 267p. Bowman,D. and Goldsmith,V. 1982. Bar morphology of dissipative beaches: an empirical model. Marine Geology 50 (in press). Chappell,J. and Eliot,I.C. 1979. Surf beach dynamics in time and space - an Australian case study, and elements of a predictive model. Marine Geology 32:231-150. Clifton,H.E., Hunter,R.E. and Phillips,R.L. 1971. Depositional structures and processes in the non-barred high-energy nearshore. J^ Sediment Petrol. 41:651-670. Exon,N.F. 1975. An extensive offshore sand bar field in the Western Baltic Sea. Marine Geology 18:197-212. Davidson-Arnott,R.G.D. and Greenwood,B. 1976. Facies relationships on a barred coast, Konchibougnac Bay, New Brunswick, Canada. S,E.P.M. Spec. Publ. No.24:149-168. Goldsmith,V., Bowman, D. and Kiley,K. 1982. Sequential stage development of crescentic bars: southeastern Mediterranean. J. Sediment. Petrol. 42:233-249. Greenwood,B. and Davidson-Arnott, R.G.D. 1979. Sedimentation and equilibrium in wave-formed bars: A review and case study. Can. J^ Earth Sci. 16:312-332. Guza,R.T. and Inman,D.L. 1975. Edgewaves and beach cusps. J. Geophys. Res. 80:2997-3012. Guza,R.T. and Bowen,A.F. 1975. The resonant instabilities of long waves obliquely incident on a beach. J^ Geophys. Res. 80:4529-4534. Hayes,M.O. 1972. Forms of sediment accumulation in the beach zone. In: Meyer,R.E.(ed.), Waves on beaches and resulting sediment transport. Academic Press, New York :297-356. Hunter,R.E., Clifton,H.E. and Phillips,R.L. 1979. Depositional processes, sedimentary structures, and predicted vertical sequences in barred nearshore systems, southern Oregon coast. J. Sediment. Petrol. 49:711-726. Inman,D.L.., Gayman,W.R. and Cox,D.C. 1963. Littoral sedimentary processes on Kanai, a sub-tropical high island. Paci f ic Sci. 17:106-130. King,C.A.M. 1972. Beaches and Coasts. Edward Arnold, London 570pp. Short,A.D. 1979a. Wave power and beach-stage : a global model. Proc. 16th CEC (Hamburg) :1145-1162. Short,A.D. 1979b. Three dimensional beach stage model. J^ Geol. 87:553-571. 105

Short,A.D. 1981. Beach response to variations in breaker height. Proc. 17th CEC (Sydney) :1016-1035. Short,A.D. 1983. Physical variability of sandy beaches, (this volume). Sonu,CJ. 1973. Thre dimensional beach changes. J^ Geol. 81:42- 64. Wiegel,R.L. 1964. Oceanographic Engineering. Prentice Hall, New York. Wright,L.D. 1981. Beach cut in relation to surf-zone morphodynamics. Proc. 17th CEC (Sydney) :978-996. Wright,L.D. 1982. Field observations of long-period, surf-sone standing waves in relation to contrasting beach morphologies. Austr. J. Mar. Freshwater Res. 33:181-201. Wright,L.D. and Short,A.D. 1982. High-energy nearshore and surf zone morphodynamics. JOA (Halifax), Abstracts, p.86. Wright,L.D., Thorn,B.G. and Chappell,J. 1978. Morphodynamic variability of high-energy beaches. Proc. 16th CEC (Hamburg), p.1180-1194. Wright,L.D., Chappell,J., Thorn,B.G., Bradshaw,M.P. andCowell,P. 1979. Morphodynamics of reflective and dissipative beach and inshore systems: southeastern Australia. Marine Geology 32:105-140. Wright,L.D. Giza,R.T. and Short,A.D. 1982. Dynamics of a high-energy dissipative surf zone. Marine Geology 45:41-62. -0

berm I--5 inner trough --10 m inner bar outer outer A. trough bar

--5 overwash --10 m bar/ berm no nearshore bars B.

Fig.xi-l. Morphodynamic beach stages. .T A. Dissipative or high energy profile. B. Reflective or low energy profile. 1:250 1:100 1:50 1:25 1:10 1:5 1:2.5 1:1 4 J, I ,gJ II I I ,l I I L-J I L I I I I I ,l I

2 4 6 8 10 20 30 45

BEACH SLOPE (degrees)

Fig.xi-2. The relationship between wave energy, beach slope and grain size. Fig.X1-3A.Typical succession of internal sedimentary structures characterizing a high-energy, dissipative beach system mean sea-level

L. '•' "r * i" ••" • i * 4.\ -^ ^i-vtc-J-r---^

Fig.X1-3B.Typical succession of internal sedimentary structures characterizing an inter-mediate energy, oblique-barred beach system. 1 ' \\.-y<: "•>*•• -

•roL.

••&•

Fig.X1-3C.Typical succession of internal sedimentary structures characterizing a non-barred, reflective beach system. 106

VTT & OPRT TMTMÏDV MnnFT rtV dPnTMPOT nTqpFRRAL

BETWEEN PORT ST JOHN'S AND THE MSIMKABA RIVER

(SOUTHEAST AFRICAN CONTINENTAL MARGIN).

by

Rowena Hay

INTRODUCTION

This study deals with detailed aspects of sediment dispersal between Port St John's and the Msikabo. River along the east coast of southern Africa (cf. Fig. I). The area is of particular interest as it extends across the boundary of two adjacent sedimentary compartments identified in a previous study,

(Flemming, 1980) (viz. compartments 3 & 4). The two compartments are separated by a major structural offset in the continental margin, which is recorded as a prominent topographic feature along the shoreline as well as the shelf break. A detailed sediment sampling programme (Fig I) has allowed a closer look at the interaction of sediment dispersal patterns between the two compartments.

PREVIOUS WORK

Regional trends in sediment distribution were previously described by Moir (1976), who divided the shelf into two, shore-parallel provinces on the basis of carbonate content. This model was subsequently refined by Flemming (1978, 1980) after extensive side scan sonar surveys. He confirmed the reqional relationship between the carbonate content of the sediments and the water depth and proposed a wave-.lominated, modern, near-shore 107 terrigenous facies which grades laterally through a midshelf, mobile sand facies into an outer-shelf carbonate-rich gravel lag environment. The latter two facies are dominated by the .

This facies distribution is due to a unique combination of

factors, the most important being shelf morphology, wave and wind driven currents, sediment supply and the Agulhas current

(Flemming, 1981). Briefly, the nearshore sand sheet is considered to be in dynamic equilibrium with the local wave

regime. The seaward edge of this wedge is periodically eroued by

the Agulhas current, which is known to migrate laterally across

the shelf from time to time (Darbyshire, 1972). The entrained

sediment is moved southwards in a sandstream on the central

shelf. Thus at any particular locality the sand wedge is either being eroded by the current or built out by offshore transport of bedload material.

The lateral extent of both the inner and midshelf facies is

a function of the position and velocity of the Agulhas current in

conjunction with the local morphology of the shelf. This is

demonstrated where the influence of the Agulhas current on the

nearshore sand sheet is inhibited by the presence of a drowned

Pleistocene coastal aeolinite ridge, which is mostly situated in

a midshelf position. In some places little sediment from the

inner shelf sand sheet is entrained by the current and a sediment wedge abutts the shoreward side of the ridge. In such areas the

outer shelf lag deposit can extent onto the middle shelf.

Where there is a major structural offset in the continental

margin, the current overshoots the shelf break, at the same time

inducing a clockwise in the lee of the offset. As the

current flows over the shelf edq*> jt deposits its nand load onto 108 the upper continental slope. A new sandstream is generated where the current rejoins the shelf further downstream. This situation describes the southern and northern limits of two adjacent sedimentary compartments respectively which is the fcc^.s of this study.

BATHYMETRY

The bathymetry of the Hast Coast has beer, compiled md discussed by Birch (1981). The oathymoti i.: map (Fig 2) was reorawi: by hand, contouring th -• South Afri.-.aa Navy Charts No's 21 and 23.

The fa thorn soundings were .neLricateu ixnc contours were then drawn at 2m intervals. These contours were smoothed and interpretations confirmed where necessary by bathymetric records.

The isobaths between 3AN21 and SAN23 i.e. north and south of the

Bluff do not coincide. There is a lOrr. discrepancy which is possibly a base-level error in the data collection or conversion.

While this is noted, it has for ail practical purposes been ignored as it would not alter any of the conclusions presented below.

From Port St John's to Msikaba River the coastline trends northeast - southwest, with a major structural offset at Waterfall

Bluff, The offset in the continental margin has been related to the onshore Egosa Fault (Birch, 1981). The shelf break is similarly offset to the west, but some distance further to the south, a feature which has yet to be explained structurally

(Flemming pers. comm.).

The gross regional pattern of the continents 1 shelf in this area is summarised in Tables T. TI, ITT. TVo area can be divided

into a nuMber of physiographic .:,• -.-v <•_'•• >vh L•.-.); tijii'-wr to be related 109

to dynamic processes active since the last major transgression.

North and south of the Waterfall Bluff lineament, the inner and

middle shelves differ, while the outer shelf remains a continuous

feature.

From the Msikaba River mouth to Waterfall Bluff the inner

and middle shelves narrow appreciably. Offshore of the Mfihlelo

Spit the inner shelf merges with the middle shelf, creating a

steep incline uown to the almost flat Waterfall Bluff Terrace.

The inner and middle shelf region changes markedly across the

offset. To the south the inner shelf developes into a narrow,

relatively steep zone as the middle shelf becomes exceptionally

broad with a very gentle slope (0.47°). At -90m it flattens

out into the southern half of the Waterfall Bluff Terrace. The

inner shelf steepens and narrows further to the south. The

middle shelf narrows in a similar fashion. It slopes evenly down

until at -100m it levels out into the Mbotyi Terrace. South of

the Mntafufu River up to the head of the Mzimvubu Canyon, the

inner and middle shelves assume a uniform slope which appears to

be a continuation of the middle shelf immediately to the north.

Between -90 and -100m the slope decreases even further. The

latter depth outlines the shoreward edge of the outer shelf

terrace.

South of the Mzimvubu Canyon the inner and middle shelf

become distinguished again by slope and width. Between the

Mzimvubu and St John's Canyons the inner shelf is relatively

broad (2.2km) and the incline gradual while the middle shelf

becomes markedly steeper and narrow again. Between the 90m and

the 100m isobath there is once more a gentle incline down to the

St John's Terrace. j While the inner und middle h'ne.'ve.s between the Msikaba River 110

and Port St John's have been shown to vary considerably, both

parallel as well as perpendicular to the shoreline, the outer

shelf remains remarkably constant in depth and slope despite the majcr offset in the shelf break.

Between the Msikaba and Port Grosvenor the outer sht-lf

slopes gradually from -80m to the shelf break at -100m. The

shelf break swings shoreward» at the Egosa Canyon. As a result

the outer shelf is narrower in this area. South of of Egosa

Canyon the outer shelf character changes to a broad, flat (5.9km)

and rectangular-shaped terrace between -90 and -100m. It e/.tends

fcr 12. 2krr. £cuthv/?.rd£: before be-inf ^v<'iin-t-"v i v torminai-pri Vw th**

offset. The position of the Mbotyi Canyon enhances the sediment

sink effect of this structural feature. The southern edge of

this terrace has ax first an unusually gentle slope which

steepens suddenly at -110m. This feature is mentioned here,

because south of the Mbotyi Canyon the shelf break is also found

at -110m, whereas to the north it is consistently situated at

-100m.

This terrace-like character of the outer shelf continues

southwards. The Mbotyi, Mzimvubu and St John's Canyons define

the northern and southern edges respectively of the Mbotyi and St

John's Terraces. These terraces are considerably smaller in

size, being outlined by the 100 and 110m isobaths. Were it not

for the canyons the terraces would appear as a single, continuous

physiographic feature extending southwards from the northern edge

of the Waterfall Bluff Terrace. This suggests that there is

possibly some sejiment exchange on the outer shelf between the

two sedimentary compartments.

Two features are suffiriently prominent to be mentioned

independently. Thti.se axe the drownoa Pleistocene ridge and the Ill

Mfihlelo Spit. The former is about 1.8km wide, 12 to 25m high and lies between 2 and 3.7km offshore with a base level at -68m.

In this region the ridge is found between the Msikaba River and

Port Grosvenor. It gradually decreases in height southwards.

The Mfihlelo Spit extends for 4km Southwards from the Bluff. It is about 800m wide and has a relief of about 20m. Side Scan sonar and shallow seismic records suggest a prograding feature

(Fig. 3). The spatial distribution of the medium, fine and very fine sand support this interpretation. Both features described above have been documented by Flemming (1981) and Birch (1981).

SEDIMENT DISTRIBUTION

Trends in the CaC03 content and the distribution of the various size fractions for the total sediment are discussed below. As only the textural data of the total sediment are presented, the patterns described cannot be conclusive. It will be shown, however, that a good indication of sediment sources, transport directions, sediment mixing and depositional processes is given.

The tvo main sources of sediment are fluvial discharge and biogenic production. The biogenic material consists predominantly of relict material entrained on the outer shelf and transported as bedload in the Agulhas current (Flemming, 1978).

The main rivers supplying sediment to the area are tne Mzimvubu and the Msikaba. The amount of material has been estimated by

Flemming (1981). If one assumes that the two rivers dominate the

6 supply then a total of 4.190 x 106m3 and 2.634 x 10 m3 is being supplied annually to the shelf south and north of Waterfall

Bluff respectively. Modern biogenic input is small in this region. At least 95% of the tn+nj sp'.l.iment from the rivers is 112 transported as suspended load and only about 5% as bedload.

CaC03 DISTRIBUTION (Fig. 4).

The relationship between terrigenous and biogenic material on the shelf is reflected in the distribution map of CaCC>3.

North of Waterfall Bluff the CaC03 describes a shore-parallel trend. From Msikaba River to the offset there is a progressive increase with water depth. Thus at -30m the CaCC>3 content is about 25%, whereas at -60m in the north and -100m in the south it increases to 75%. North of Port Grosvenor the 75% contour trends parallel Lo Lhe uuLir edge of the middle ehilf a? H^fin^d by

Flemming (1980). To the south of this point this boundary coincides more nearly with the 50% contour. This seaward shifting of the contours is physiographically controlled as it is strongly influenced by the presence or absence of the drowned

Pleistocene sediment ridge.

South of the Mbotyi Canyon a uniform increase in CaCC>3 with water depth is observed only on the outer shelf. The 50% contour lies approx. parallel to the shelf break at -100m, while

the 75% contour does not extend beyond th>» offset. South of the

Waterfall Bluff the 25% contour parallels the mid to outer shelf

transition (-100 to -110m). North of the offset, on the seaward

side of the Mfihlelo Spit, this contour swings sharply across the

shelf, trending northwards as described above.

The 20% contour outlines the spit and then continues

southward parallel to the coast at -30 to -40m as far as the

Mzintlava River. Off this river mouth it crosses the middle

shelf and continues north for a short distance before continuing

south parallel to the 25% contour on the middle shelf. The 10% 113 contour is localised around the Mzimvubu sediment wedge. It extends across the inner shelf to the head of the Mzimv.:b'j Canyon before swinging shorewards.

Sirall discrete areas of high CaC03 content occur on the middle shelf south of Waterfall Bluff. Such areas represent

'windows' and indicate that the coarse palimpsest facies continues below the terrigenous sediment wedge. It is pure coincidence that such areas were sampled and the phenomenon is therefore likely to be more common than is indicated on Fig. 4.

VERY COARSE SAND DISTRIBUTION (Fig. 5).

Very coarse sand occurs in small amounts only. On the middle and outer shelf to the north of the offset it reaches concentrations of just over 5%. South of the offset on the outer shelf it barely exceeds 1%, whereas the inner and middle shelves have less than 1% or nothing at all.

COARSE SAND DISTRIBUTION (Fig. 6).

The contours of the coarse sand fraction essentially follow the same regional trend as those for very coarse sand. North of

Port Grosvenor the contours run closely to the shoreline, with the 50% contour being situated at about -40m. Immediately south of Port Grosvenor they swing dramatically across the shelf. The

50% contour now rapidly crosses the northern edge of the

Waterfall Bluff Terrace and terminates at the shelf break, whereas the 10% line parallels the eastern edge of the middle

shelf at -90m. The 10 and 25% contours both terminate at the

southern edge of the terrace. The 1% contour moves persistently

southwards along the -, -•.'-»der: ;:'; .<.tw<-en -60 and 114 -100m, as far as Port St John's. Patches of coarse sediment in amounts greater than 5% are occasionally recorded on the outer shelf terraces. Immediately to the south of Port St John's the outer contours move inshore again. Here between 1% and 5% coarse sand is found on the inner and middle shelves, with 5 to 10% recorded on the outer shelf.

MEDIUM SAND DISTRIBUTION (Fig. 7).

The medium sand fraction has a similar regional distribution to that described by the coarse and very coarse sand. At Waterfall Bluff the 5%, 10% and 25% «.-un Lours gradually move offshore to depths between -30 and -50M. They swing around the Mfihlelo Spit and continue south along the seaward edge of the middle shelf (-100m). On the outer shelf, North of Waterfall Bluff, there is generally more than 50% medium sand. Sediment containing more than 50% medium sand does not occur on the outer shelf between Mbotyi Canyon and Port St John's. At Port St John's, however, a belt of medium sand stretches across the shelf to the St John's canyon head. On either side of this corridor the medium sand contour drops rapidly to zero, with the exception of a nearshore belt wich maintains concentrations in excess of 10%. Higher amounts are recorded in the geological windows referred to earlier. Elsewhere on the middle shelf there is less than 5% medium sand in the sediment. Insufficient data are available at present for the area south of Port St John's. The amount of medium sand on the inner and middle shelves appears to decrease southwards until a similar trend to that found north of Waterfall Bluff is re-established.

A comparison between distribution patterns of medium sand, 115 coarse sand and very coarse sand suggests a close association of these fractions over most of the study area. This means that the localised high concentrations of medium sand along the inner shelf between the Mntafufu and Mbotyi livers might also be interpreted as originating in the geological windows observed along the inner edge of the middle shelf. The CaCC>3 map, in fact, supports this interpretation. An exception would be the medium sand corridor off Port St John's which appears to have its source in the Mzimvunu River.

FINE SAND DISTRIBUTION (Fig. 8).

The fine sand component has an inverse relationship to the trends described above. The amount of fine sand comprises up to

75% of the total sediment in the nearshore region (-20 to -30m).

This decreases to less than 5% on the outer shelf north of the offset. Between the Msikaba River and Port Grosvenor the concentration gradient is steep. At approx. -40m the proportion of fine sand has dropped to less than 5%. At Port Grosvenor the contours shift seawards. The 25% contour now follows the outer limit of the middle shelf(-90m) with a progressive decrease to less than 5% at approx. -95m. The 50 and 75% contours remain between -20 and -30m. The central shelf sancistream in this area thus carries at least a 2 5% proportion of fine sand.

In the lee of Waterfall Bluff the 75%, 50% and 25% concentration lines faithfully contour +he Mfihlelo Spit and then continue southwards along the inner shelf. Up to 25% fine sand occurs on the middle and outer shelves of this region. Off the

Mzimvubu River the fine sand fraction follows a similar trend to that cf the medium sand by also cut!, v^g across the shelf, although it is s.-'-n-'-w^-it. o' £:.••< -.•• ' r th. On the central 116 shelf to the north and to the south of this corridor the proportion of fine sand drops below 25%. In general, areas with high concentration of fine sand correspond to areas of low concentration in medium and coarse sand, the only exception being the corridor off Port St John's and some degree of overlap with the medium sand province of the sandstream in the north.

VEKY FINE SAND DISTRIBUTION. (Fig. 9).

Two distinct trends emerge from the distribution of very fine sand. North of Waterfall Bluff the proportion of very fine sand decreases rapidly with water depth. Very fine sand is absent on the outer shelf of this region, except on the shoreward side of the Waterfall Bluff Terrace where it is associated with the eddy. South of Waterfall Bluff the exact reverse is found. Very fine sand now increases with water depth and is mostly concentrated on the middle shelf. Off Port St John's the corridor, occupied by medium sand and fine sand, is also outlined by the very fine sand distribution, but now as an area of low concentration.

DISCUSSION AND CONCLUSION

On the basis of the sediment distribution patterns described above a preliminary model of sedimet dispersal on the continental shelf between the two adjacent sedimentary compartments can be constructed.

To the north of the Offset medium sand, with greatest concentrations on the middle and outer shelf, appears to form the main bedload component of the centra]-shelf r.and stream. All of this sediment, including any finer sediment, is eventually dumped 117 onto the upper continental slope at the offset as the current overshoots the shelf break (Flemming, 1980). On the ether hand the distribution patterns of coarse and very coarse sand of the same region do not suggest much southerly movement. It forms a pavement of mostly relict lag material deposited during the

Flandrian transgression.

Fine sand is concentrated in the nearshore zone of the northern shelf sector, where a deposit of considerable thickness has accumulated. The origin of this material is uncertain, although a strong case can be made for a source situated to the south or the offset - at least for a substantial proportion of this material. This conclusion is reached on the basis of the corresponding distribution patterns of individual size fractions on the shelf to the south of Waterfall Bluff.

Between Port St John's and Waterfall Bluff the presence of medium, coarse and very coarse sand in appreciable amounts is limited to isolated occurrences on the middle shelf and at the shelf break. The former has been interpreted as representing windows of the same relict material extensively found on the outer ahelf to the north of the offset. The presence of coarser, carbonate-rich material on the outer shelf terraces in this region suggests a marginal influence of the i.gulhas current as it nicks the shelf edge. Fine sand in this area is concentrated in a narrow zone on the inner shelf while very fine sand mantles the middle and outer shelves shoreward of the current-swept terraces.

The distribution of the coarse, medium, fine and very fine sand fractions offshore of the Mzimvubu River differs from the regional pattern described above. Across the shelf towards the head o' fVie St John's Canyon a corridor of hiqh medium sand conceii'.r.-iti&ri ov^rl.ijs '.v. th , -.-•• .;k,. _''.••/ ?.: v :y fine sand. 118 South of this belt medium sand is concentrated in anounts between 20% and 50% of the total sediment. Little medium sand occurs to the north. This means that a large proportion of the medium sized fraction supplied by the Mzimvubu River is being carried across the shelf and lost down the head of the St John's Canyon. It is suggested that when the Mzimvubu River is in full flood, the strength of the flow is sufficient to transport medium-sized material as temporary suspension across the shelf, where some escapes down the St John's Canyon. The remainder is entrained by the Agulhas current and carried southward. The renewed influence of the current is supported by the fact that the coarse sand component increases rapidly ncrccc the shelf to the south of this corridor.

In analogy to the medium sand, much of the fine sand is also transported across the shelf, but towards both the Mzimvubu and the St John's Canyons. The remainder, together with a very fine fraction is carried northwards by the eddy flow induced by the Agulhas current in the lee of the offset. This northerly movement of sediment is reinforced by the frequent (60% p.a.) occurrence of southwesterly gales which generate surface waves and nearshore wind-stress currents counter to the Agulhas current. As it is the Westerlies which bring rain to the hinterland, it explains how the present distribution of sediment has persisted through time. Birch (1981) reports up to 34m of sediment across the shelf on the Mzimvubu river and up co 5 and 10m of material draped over the canyon heads.

As the sediment is moved north the finer sediment is deposited in the nearshore zone, while the very fine material moves into the centre of the eddy to cover the middle and marginal outer shelves. In the lee of the bluff the fine sized 119 material is either deposited on the spit or moved north by

littoral drift. In the north it is deposited in the nearshore zone, from where it gradually moves across the shelf by wave action until it is entrained in the sandstream and deposited over the shelf break at the offset. Very little of the very fine material is present north of the bluff. some of this material settles out on the seaward edge of tne Mfilelo Spit, whereas some is dump-d in the head of the Mbotyi canyon. The largest portion however, appears to resirculate in the eddy, being deposite on the middle she I?' and on the inner edges of the outer shelf terraces to the south of Water rail fiiutf where the energy is

lowest.

The above pattern of sediment dispersal which is summed up by the mean diameter map of Fig 10 draws a complex picture of regional separation and local mixing of different populations.

Further detail might be revealed by evaluating the trends of the terrigenous and the biogenic sediment components separately.

ACKNOWLEDGEMENTS

I am grateful to Mr Matthew Smith who showed a grsat deal of

patience and good humour while being generous with his assistance

in the sediment laboratory.

Thanks are also rue to Bob de Deckei , Henri Fortuin and

Keith Martin who were at all times prepared to discuss and

elaborate a point.

Finally I wish to thank Dr Burg Flamming without whose

expert guidance this report would not have been written; also Rob

Post arw1 Dr Chris Hartnady, without whose word-pz ocessing efforts

this report would not have ...;{.••• ,••• • r>t\ . . 120

REFERENCES

Birch,G.F. 1981. The bathymetry and geomorphology of the continental shelf and upper slope between Durban and Port St Johns. Ann. Geol. Surv. S. Afr. 15/1:55-62.

(in press ). Sedimentological and geophysical investigation at a major sediment exit point on the southeast continental margin (vicinity of Port St Johns). Geol. Surv. S. Afr. Bull.

Darbyshire,J. 1972. The effect of bottom topography on the Aguihas Current. Pure Appl. Geophys., 101:208-220.

Flemming,B.W. 1977. Depositional Processes in and Lange- baan Lagoon. CSIR Research Report No.362. 215pp.

1978. Underwater sand dunes along the southeast continental margin - observations and implications. Marine Geology, 26:177-198.

1980. Continental margin sediment dynamics I. A generalised model of material transfer. Joint GSO/UCT Marine Geoscience Unit, Tech. Rept. No.11:49-56.

1980. Sand transport and bedform patterns on the continental shelf between Durban and Port Elizabeth. (Southeast African Continental Margin). Sed. Geol., 26:179-205.

1981. Factors controlling shelf sediment dispersion on the south east African continental margin. Mar. Geol., 42:259-277.

Moir,G. 1976. Preliminary textural and compositional analyses of surf- icial sediment from the upper continental margin between Cape Recife (34°S) and Porto Do Ouro (27°S) South Africa. Joint GSO/UCT Marine Geoscience Unit, Tech.Rept. No.8:68-75. 121

LIST OF TABLES

Table I: Variations in the continental morphology between the Msikaba River and Port St Johns.

Table II: Variations in the outer shelf terraces north and south of Waterfall Bluff.

Table III: The influence of canyon heads on the continental shelf morphology. Physiographic Inner Shelf Middle Shelf Outer Shelf Ave 6ot ton) Bolt Om Slope shelf " Area W dth Depth Width Slop» Daf.th Width S lope DdOth width (km) degrees (m) (km) degrees (m'j (km) Uegr* PS (m) 'km) Msikaba River

ÍO 3,2 1,0 -50 2,2 1,0 -i0 3,7 0,15 -100 9,1 Port Grosvenor

Port Grosvenor •

to 3,0 0,78 -90 -^ 5,4 0,12 -100 8,4 Waterfall Bluff

Waterfall Bluff

to 5,8 0.55 -50 5,4 0,47 -90 6,0 0,21 -110 17,2 Mzintiava River

Mzintiava River -100 0,23 -110 6,4 to 2,2 2,75 -95 1.4 0,23 2,8 Port St. Johns

TABLE T Table j[ max width (km) N/S length (km) Waterfall Bluff 5,9 13,2 Terrace

Mbotyi 3,5 9,6 Terrace

Si John's ?,8 6,5 Terrace

Table HI Head distance width of width of from shore shelf to N. shelf to S.

EGOSA 7,4 km 8,8km 9,1 km CANYON

MBOTYI CANYON 6,6km 8,8km 8,1km

MZIMVUBU CANYON 4,0km 8,1km 7,6km

ST JOHNS CANYON 4,9km 6,6km 6,6km LIST OF FIGURES

Fig. 1 Locality map with sample locations.

Fig. 2 Bathymetry between the Msikaba River and Port St John *s

Fig. 3 Seismic section of the Mfihlelo Spit.

Fig. 4a CaC03 distribution on the shelf.

Fig. 4b Distribution of terrigenous material on the shelf.

Fig. 5 Distribution of very coarse sand fraction.

Fig. 6 Distribution of coarse sand fraction.

Fig. 7 Distribution of medium sand fraction.

Fig. 8 Distribution of fine sand fraction.

Fig. 9 Distribution of very fine sand fraction.

Fig. 10 Distribution of mean diameter variation on the shelf. a£a.

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XIII PLEISTOCENE PHOSPHORITES OFF THE WEST COAST OF SOUTH AFRICA by G.F.Birch, J. Thomson, J. McArthur and W.C.E.urnett

INTRODUCTION Holocene phosphate mineralization is well documented for siliceous facies (authigenic) phosphorites from areas of intense coastal upwelling and high biogenic activity off Peru/Chile (Burnett, 1977; Burnett and Veen, 1977) and off South West Africa () (Baturin et al, 1972; Veen et al, 1974; Baturin, 1972). Recently, radiometric data (O'Brien and Ve»»h, 1980; Kfees and Vaeh, 1980) have indicated Quaternary phosphatlzation on the east Australian margin under conditions of moderate, seasonal upwelling and low primary productivity. Uranium series age dating of samples from the calcareous facies (replacement) phosphorite province on the western margin of South Africa suggests that phosphatization of palaeontolog'.cally old (Mid Eocene - Mid Miocene) limestones has occurred at luast as recently as the late Pleistocene (62 - 76 kyrs). Overprinting by subaereal leaching and readsorption of U during euistatic sealevel fluctuations following substantial leaching is considered unlikely based on geochemical evidence.

Extensive areas of the upper continental margin off South Africa and South West Africa (Namibia) are mantled by phosphatic sediments (Murray and Renard, 1891; Baturin, 1969; Senin, 1970; Parker, 1975; Birch, 1977; Birch, 1979; Rogers, I9"i7; Bremner, 1977). A broad (^75 km), near-continuous "pavemenu" of phosphorite exists on the southern and eastern Agui.has Bank and in a narrow zone (^25 km) located near the shcj.£ break on the south western margin (Parker, 1975) . Extensive, discontinuous phosphatic deposits occur northwards to Luderitz (lingers, 1977), 123 but between Sylvia Hill and Walvis Bay large parts of the shelf are again covered in phosphatic sediment. Discontinuous concentrations persist to at least as far north as the Kunene River (Bremner, 1977a; 1977b; 1980). A regional differentiation exists between siliceous facies phophorites and calcareous facies phosphorites. The former genetic types occur north of Sylvia Hill as oolitic and structureless pellets on the middle shelf and as concretions and laminae associated with diatomaceous muds on the inner shelf (Bremner, 1977a; 1977b; 1980). Replaced calcareous rocks dominate the shelves south of Sylvia Hill» Authigenic quartzitic packstones and pelletal phosphorite are, however, located onland at Saldanha Bay. Natural series data on siliceous facies phosphorites from the South West African (Namibian) margin, where upwelling is Intense, have been conclusive in demonstrating Recent phosphorite formation. Soft or friable concretions contained in the diatom ooze have 234y/238u acti"ity ratios close to the modern seawater value with estimated ages up to 25 kyr (Baturin et al, 1972). In these concretions uranium and P2O5 increase together, and some examples contain modern diatoms (Baturin, 1972)* Light coloured, unconsolidated phosphatic laminae are dated as contemporaneous on

230Th/234u and 234U/238U activity ratio evidence (Veen et al, 1974), whereas black pellets and grey nodules are beyond the age of the method, i.e. >700 kyr. No firm data are available for the middle shelf pelletal phosphorites from tiiis locality, but they are regarded as Miocene (Bremner, 1977).

In contrast to the Namibian samples, ideas on the age of the phosphorite on the South African shelf have come mainly from palaeontological and stratigraphic evidence. Early workers (Murray and Renard, 1891; Collet, 1905) regarded the Agulhas Bank phosphorite as "related to present day phenomena" but later 1 *>" macro- and micro-faunal evidence (Cayeux, 1934; Haughton, 1956) suggested a Miocene age. From stratigrnphical relationships (Dingle, 1975) ami by associations w"th other rock types (Parker and Siesser, 1972) two main phases of phosphatisation were postulated for those rocks; Upper Eocene and late Miocene/early Pliocene or late Pliocene. A comprehensive foraminiferal and nannofossil examination of calcareous facies phosphorites from 33 stations (Siesser, 1978) confirmed a Mid Miocene to Pliocene fossil age, and this range was supported by later paleontological work (Birch, 1979; Rogers, 1977). Uranium isotope (Kolodny and Kaplan, 1970) and electron microscope (Baturin and Dubinshuk, 1974) studies added other lines of evidence militating against a Recent phase of phosphate mineralisation. Considerable evidence from divergent fields has therefore provided evidence against phosphate mineralisation on the South African margin since the Pliocene. The new evidence presented here advocating a late Pleistocene phase thus represents a radical departure from accepted concepts.

METHODS Uranium series age dating was conducted at Florida State University (FSU) and the Institute of Oceanographic Sciences (IOS) by established methods (Burnett and Veen, 1977; Veeh et al, 1974; Kolodny and Kaplan, 1970) based on alpha spectrometry. Three phosphorites, a quartzose packstone (GB-1) and two phosphatic limestones (310 and 2246) were analysed comprehensively. They were recovered from the middle shelf off Cape Town from depths of 315-360 m. The latter two rocks were selected because they have been firmly dated on nannofossil evidence as Mid Eocene to Mid Miocene (50 to 12 million years) (Siesser/ 1978). Different approaches were taken to sample dissolution by the 125 two radiochemical laboratories. The FSU procedure was designed to remove all phosphatised material leaving the resistant detrital phase, whereas the IOS procedure was based on total dissolution.

RESULTS The consolidated data for the three samples are presented in Table la along with comparative data for the N.B.S. standard 120B (Florida Phosphate Rock). The uranium and thorium contents and

the 230Th/232Tn activity data, produced by FSU are slightly lower than those from IOS. This is considered to be due primarily to the differences in dissolution procedures, as the bulk of the thorium, but only a small amount of the uranium is in the detrital phase. However, similar values are obtained for the

234u/238u an(j 230Th/234u activity ratios, and it is these which are used in age calculations. Some variation in the data is due to the two laboratories analysing different splits of an unhomogenised sample. Nevertheless, the isotope activity ratios are in excellent agreement and are within experimental error. Corrections for detrital or initial 230Th (Burnett and Veen, 1977) cause relatively small adjustments to calculated ages (sample GB-1 is an exception) due to the high uranium content being offset by high U/Th ratios for the two phosphatic 1imestones. An important further corroboration to the ages estimated

from th*. 230Th/234u activity ratios is provided by 231Pa/235u (Table lb). These analyses were performed by a proxy method

227 231 which determines Th as a measure of Pa (Mangini and Sonntag, 1977; Oe Master, 1979). An agreement of the 230Th/234u

and 231pa/^35y methods has been demonstrated for (Veeh, 1982) and recently for phosphorites (Veeh, 1982). A highly satisfactory correlation between the two methods for the 126 phosphatic limestones lends support to a single episode of uranium uptake, as two (or more) phases of uptake would tend to make the 231Pa/235U ages systematically lower than those from 230Th/234u because of the different half lives of 231Pa (34 300 years) and 230Th (75 200 yearJ). It may be argued that some correction for a detrital or initial 231pa content should be made, similar to that used for 230Th, but there is at present no available data for this adjustment. The age estimates for sample GB-l do not give such good agreement as for the phosphatic limestones, as the 230Th/23^u, corrected 230Th/234U and 231pa/235U ages are not in concordance. It is unclear whether or not any of these are correct, and we note only that the 23iPa/23^U age agrees with those estimated for the nearby sample 310.

DISCUSSION A marked disparity therefore exists between the paleontological and uranium series ages for these samples. The nannofossil evidence is not in doubt and is supported by dates from other fauna (foraminifera, mollusca and brachiopoda) (Collet, 1905; Cayeux, 1934; Siesser, 1977). Similarly the uraniur.-. series data have been shown to be consistent for the phosphatic limestone samples which are particularly suited to the technique. It is concluded therefore that recent incorporation of uranium into these ancient limestones has occurred. However, uranium-series disequilibrium method of dating are not applicable to phosphorites which have been weathered or secondarily enriched in uranium as U-series isotopes do not form a closed system in apatite under such conditions (McArthur, 1978). It therefore remains for us to demonstrate that uranium incorporation is contemporaneous with phosphatisation.

T.ie ratios for apatite-S04/P205 and apatite-Na/P204 and the 127

content of apatite-bound C02 (Table 2 (Burnett and Veen, 1977; Baturin et al, 1972)) are within the range for unaltered marine phosphorite (Parker and Siesser, 1972; McArthur, 1978). The

Sr/P205 ratio for GB-1 is about average for sea-flcor phosphates whilst 2246 is about 25% higher and 310 is about 15% higher. This may reflect the effect of retention of Sr in accessory in 2246 and 310 (Table 2). It is interesting, however,

that Sr/P205, U/P205 and F/P205 ratios (Table 3) vary sympathetically in these samples and both show an inverse

correlation with calcite content (i.e. total C02) in the rocks. The highest ratios occur in the oldest sample (2246). Whether this results from continued uptake of U, F and Sr after the major phosphogenic episode cannot be decided on available evidence, but the variability in ratios does suggest that the dates presented are 'accumulated' dateb averaged over a period which may be a significant fraction of the date. Preliminary studies of soft, friable phosphatic material separated from voids in conglomeratic phosphorite from the South African continental margin give ages of 130 kyr (O'Brien, personal communication, 1982), thereby adding credence to a concept of a range of accumulated ages. The carbon and oxygen isotopic compositions of the apatite-

bound C02 (Table 1) are within the range found in normal marine limestones and confirms that the apatite in these samples was formed by phosphatisation of a carbonate precursor . They are heavier, however, by about 0.5°/oo in 13C and l°/oo in 180 than other Agulhas Bank phosphates . This coherent difference is commensurate with our proposal that, unlike most Agulhas Bank phosphates, the apatite in these phosphates is geologically youthful. We are therefore satisfied that all available mineralogical, geochemical and age data indicate recent phosphatization of palaeontologically ancient limestones has taken place and that 128 these data do not merely represent a modern overprint event. Several questions are evoked by- the results of this investigation. The three dated samples presently being studied are of the non-conglomeratic type of phosphoritw (Parker and Siesser, 1972). However, nothing is yet known about the age of the more common phosphorite variety on the South African continental margin, i.e. the conglomeratic phosphorites (Parker, 1975). It is conceivable that previously obtained dates of >700 000 yrs were obtained from this rock type. Until recently, modern phosphate mineralization was considered to be restricted to areas of intense upwelling and high biological productivity. Quaternary phosphorites have now been reported (O'Brien and Veeh, 1980) from the east Australian margin in regions of low nutrient concentration and poor productivity. Thus, this is only the second reported occurrence of phosphorites forming in modern times outside areas of intense coastal upwelling. The process whereby carbonate fluorapatite "replaces" calcium carbonate is not well understood. Nevertheless, if a concentration of only 1 g at. P/% in saline solution is sufficient to initiate the replacement process (Ames, 1959), then contemporary phosphatization should in fact be expected on this margin of moderate upwelling where up to three times the minimum PO^-^ concentration (up to 3 g at.P/*) has been observed (Hart and Currie, 1960; De Decker, 1970). Bacteria possibly facilitate interstitial phosphate enrichment in east Australian shelf sediments (O'Brien et al, 1981) and other microorganisms are al3o considered co enhance phosphate levels (McConnell, 1965). Contemporary phosphatization may be possible in areas of moderate upwelling if non-hydrographic factors play an important role in the process. The range of the three age dates neatly straddle an eustatic trangression/regression cycle on a curve constructed by Bloom and 129 others (Bloom et al, 1974), thus supporting a previous suggestion

(O'Brien and Veeh, 1980) of continuous n.ineral ization rather than a mechanism related to high sea level stands.

ACKNOWLEDGEMENTS

The work at Florida State University (FSU) was supported by

NSF grant OCE-8007047. K. Roe am'. M. Beers assisted in the laboratory at FSU. At the University College London R. Benmore undertook the F, ?2®s anc* isotope analyst's and A. Osborne assisted with the chemical analyses. The: present work .s partly a contribution of Project 156 (phosphorites) of the International

Geological Correlation Program.

REFERENCES

Ame:3,L.L. 1959. The genesis of carbonate apatites. Econ. Geol . 54:829-841.

Baturin,G.N. 1969. Authiyenic phosphate concentrations in Recent sediments of the Southwest African shelf. Dokl. Earth Sci. Sect. English Translation 189:227-230.

1972. Phosphorus in interstitial waters of sediments of the southeastern Atlantic. Oceanology 12:849-855.

and Dubinshuk,v.T. 1974. Microstructures of Agulhas Bank phosphorites. Mar. Geol. 16:M63-M70.

, Merkulova,K.L. and Chalov,P.I. 1972. Radiometric evidence or recent formation of phosphatic nodules in marine shelf sediments. Mar. Geol. 13:M37-M41.

Birch,G.F. 1975. Sediments on the continental margin off the west coast of South Africa. Ph.D. thesis Geol. Dept., Univ. Cape Town. 1-210.

1977. Surficial sediments on the continental margin off the west coast of South Africa. Mar. Cool• 23:305-337.

1979. Phosphatic rocks on the western margin of south Africa. J. sedim. Petrol. 49:93-110.

1980. A model of contemporaneous phosphatization by diagenetic and authigenic mechanisr*-; from the western margin of southern Africa. :3EPM Spjcc^ Publ. 29:79-100. 130

Bloom,A.L.f Broecker,W.3., Chappell,J.M.A., Mathews,R.V. and Mesollella,U.S. 1974* Quaternary sea level fluctuations on a tectonic coast: new 230Th/234u dates from the Huon Peninsula, New Guinea. Qua^ern. Res. 4:185-205.

Bremner,J.M. 1977a. Sediments on the continental margin off South r

1977b. Preliminary results on the trace element geochemistry of phosphorite and glauconite from the continental shelf off Southwestern Africa. Tech. Rep, jt geol. Surv ./Univ. Cape Town mar. Geol. Progm. 9:42-50.

1980. Concretionary phosphorite from S.W. Africa. J. geol. Soc. London. 137:773-786. Burnett,W.C. 1977. Geochemistty and origin of phosphorite deposits from off Peru and Chile. Bull. geol. Soc. Am. 88:813-823.

and Veeh,H.H. 1977. Uranium-series disequilibria in phosphorite nodules from the west coast of South America. Ceochim. cosmochim. Acta. 41:755-764.

Cayeux,L. 1934. The phosphatic nodules of the Agulhas Bank. Ann. S^ Afr. Mus. 31:105-136.

Collet,L.W. 1905. Les concretions phosphatees de l'Agulhas Bank. Proc. Roy. Soc. Edin. 25:862-893.

De Decker,A.H.B. 1970. Notes on an oxygen-depleted subsurface current off the west coast of South Africa. Investl Rep. Div. Sea Fish. S. Afr. 84:1-24.

De Master,D.J. 1979. The marine budgets of silica and {32}Si. Ph.D. thesis, Yale Univ., New Haven. 1-308.

Dingle,R.V. 1975. Agulhas Bank phosphorites: a review of 100 years of investigation. Trans, geol. Soc» S. Afr. 77:261- 264. Hart,T.J. and Lurrie,R.I. 1960. The Benguela Current. Discovery Reps , 31:l.'.3-298.

Haughton,S.H. 1956. Phosphatic-glauconitic deposits off the west coast of South Africa. Ann. S. Afr. Mus. 42:329-334.

Kress,A.G. and Veeh,H.H. 1980. Geochemistry and radiometric ages of phosphatic nodules from the continental margin of northern New South Wales, Australia. Mar. Geol. 36:143-147.

Kolodny,Y. and Kaplan,I.R. 1970. Uranium isotopes in sea-floor phosphor tes. Geochim. cosmochim. Acta 34:3-24. ?"*!

Ku,T.L. 1968. Protactinium 231 method of dating from Barbados Island. J^ geophys. Res. 73:2271-2286.

Mangini,A. and Son:itag,C. 1977. 23iP^ dating of deep-sea cores via 227Tt. counting. Earth planet. Sci. Lett. 37:251-256.

McArthur,J.M. 1978- Systematic variations in the contents of Na, Sr , CO3 and SO4 in marine earbonae-fluorapatite and their relation to weathering. Chew. Geol. 2i:89-112.

McConnell,D. 1965. Precipitation of phosphates in sea water. Econ. Geol . 60:1059-1(562.

Murray,J. and Renard,A.F. 1391. Deep-sea deposits. Rep, sci. Res. H.M.S. Challenger (1873-18"6;• H.M.S.O, London.

O'Brien,G.W. and Veeh,H.H. 1960- Hoiocene phosphorite on the East Australian continental margin. Nature 288:690-692.

, Harris,J.P. ,. Milnes,A.R. and Veth,H.H. 1981. Bacterial origin of East Australian continental margin phosphorites. Nature 294:442-444.

Parker,R.J. 1975. The petrology and origin of some glauconitic and glauco-conglomeratic phosphorites from the South African continental margin. Jj_ sedim. Petrol. 45:230-242.

and Siesser,W.G. 1972. Petrology and origin of Some phosphorites from the South African continental margin. J. sedim. Petrol. 42:434-440. Rogers,J. 1977. Sedimentation on the_ continental margin off the Orange River and the NamiE~Dë"se"rtT Ph .D. tfhesis, Geol. Dept., Univ. Cape TowrH 1^211.

Senin,Yu,M. 1970. Phosphorus in bottom sediments of the South West African shelf. Lithology and Mineral Resources. 25:8- 20.

Siesser,W.G. 1977. Biostratigraphy and micropalaeontology of continental margin samples. Tech. Rep, jt geol. Surv./Univ. Cape Town mar. Geol. Progm. 9ÏT08~lT7^

1978. Age of phosphorites on the South African continental margin. Mar. Geol. 26:M17-M28.

Veeh,H.H. 1982. Concordant 230Tn and 2'3la ages of marine phosphorites. Earth planet. Sci. ^ett^ 57:278-284.

, Calvert,S.E. and Price,N.B. 1974. Accumulation of uranium in sediments and phosphorites on the South West African shelf. Mar. Chem. 2:189-202. TABLE la RADIOCHEMI CAL ANALYSES

230 234 23 226„ Sample Laboratory Tmhw u °Th 231Pa Pa u Th 232 234 235 . 238 ppm ppm mlTh U u u Th activity activity activity activity activity ratio ratio ratio ratio ratio

NBS-12ÚB FS'J 1 23- 7.9± 51-2* 1.04-0.01* 1.00^0.01* 0.98-0.04* - NBS-120B I OS 132^1* 9.1^0.3- 45.0^0.8* 1.05-0.01* 0.99-0.01* 0.97-0.04* -

GB-1 FSU 96.6- 4.3- 36^2 1.12-0.01 0.53-0.01 - - GB-1 I OS 67.3-0.9* 4.8-0.1* 33-1* 1.10-0.01* 0.53-0.01 0.72-0.04* -

310 FSU 123± 3.0± 62-13 1.09-0.04 0.45-0.03 - 0.86-0.04 310 I OS 115Í1 4.1-0.2 43.2-0.5 1.10-0.01 0.46-0.01 0.73^0.03* -

2246 FSU 269- 2.4Í 180^32 1.10-0.01 0.49-0.01 - 0.87^0.02 2246 I OS 264-3 1.9-0.1 235-14 1.10-0.01 0.51^0.01 0.78^0.02* -

•Weighted mean of more than one analysis TABLE lb ESTIMATES OF SAMPLE AGE

230, 231 230 Sample Laboratory Th Pa Th 234 235 0 u 234U age (Kyr.)* age (Kyr.) age (Kyr.)*

+2.4 GB-1 FSU 80.5 -2.2 +2.4 +7.6 IOS 80.7 63.0 74.2 •2.3 -6.6

+6.2 FSU 64.3 310 -5.9 +2.1 +5.8 IOS 66.1 64.8 61.5 -2.0 -5.2

+2.1 2246 72.2 FSU 2.2 +2.2 +4.8 IOS 76.4 74.9 75.9 -2.2 -*.3

•Calculated from data on Table 1a. 230 230 232 230 232 +Corrected Th activity used, i.e. = total Th - (4 x Th. exp(-X )t). Th and Th are activities in 230 n d.p.iiu/g, *230 is the decay constant of Th, and t is the time elapsed since formation. TABLE 2

Chemical and Stable Isotopic Composition of Phosphates

Element 2246 GB1 310

12.1 45.6 26.7 SÍO2

1.96 3.95 3.45 A1203

1.25 2.00 2.30 Fe203 (Total)

MgO 0.90 1.00 1.00

CaO 46.0 25.5 37.0

0.03 0.53 0.64 K20

0.60 0.70 0.72 Na20 (1)

P 0 11.9 14.6 14.8 2 5

C02 (4) 23.5 5.5 11.2

S04 (1) 0.90 1.00 1.00

F (1) 1.40 1.39 1.54

Sr (1) 1240 1180 1380

Total (less 0 F 99.9 101.1 99.7 and with S0„ as SO,) 4 3

Na20 (2) 0.17 0.19 0.26

S04 (2) 0.14 0.11 0.13

C02 (3) 5.7 4.8 4.9

UC •/.. (PPB) +0.77 +0.62 +0.80

18 0 °/00 (PPB) +3.37 +3.66 +3.78

(1) Elements soluble in dilute HC1 (2) Na„0, SO. soluble in distilled water 2 4 (3) Structual-CO by Gulbrandsen's method (Gulbrandsen, 1970) (4) Total C as CO TABLE 3

Element/P^O weight percentage ratios for apatite snbstituents

Ratio Sample 2246 GB1 310

0.118 0.095 0.104 P2°5

so4 X 103 64 61 59 P2°5

sr 4 X 10 104 81 93 P2°5

Na20 X 102 36 35 31 P2°5

U 4 X 10 23 5.5 8.1 P2°5 132

XIV. GUIDE TO THE SEDIMENTOLOGICAL USAGE OF THE COULTER COUNTER MODEL TAII, AT THE UNIVERSITY OF CAPE TOWN by F. Camden-Smith and A.K. Martin

INTRODUCTION

The University of Cape Town's Model TAII Cot Iter Counter has been used to count mainly monosized particulate matter in seawater.Here we describe a method of using the Coulter Counter to obtain textural analyses of mud fractions of taarine sediments. The wide size distribution of naturally occurring sediments has necessitated the use of a two tube technique. This guide must be used in conjunction with the two manuals available.

1. Operator's manual for the Coulter CounterR Model TAII 1975 (Coulter Electronics, Inc., 590 West Twentieth Street, Hileah, Florida 33010) familiarly known as the "American" manual and the one most used by ourselves.

2. Instruction manual for Coulter CounterR Model TAII 1976 (Coulter Electronics Ltd., Coldharbour Lane, Harpenden, ) known as the British manual and useful mainly for its multitube techniques. While these analyses were being undertaken, the Coulter Counter was situated in the Ecology Laboratory of the Oceanography Department, a room which was as near dust free and as clear of extraneous electrical background noise as possible. The Coulter Counter is now housed on floor A of the Zoology Department.

THEORY OF COULTER COUNTER OPERATION

"The Coulter CounterR Model TAII determines*, the number and sizes of particles suspended in a conductive fluid by forcing the 133 suspension to flow through a small aperture, and monitoring an electrical current which also passes through the aperture. As a particle passes through the aperture, it changes the resistance between the electrodes. This produces a current pulse of short duration having a magnitude proportional to the particle volume. The series of pulses is electronically scaled and counted." (American manual p.7-1). Although originally developed to count blood cells very rapidly, the Coulter Counter can count a variety of particle types. It provides a relatively accurate means of determining the size distribution of particles ranging in size from 0.64um to 560um. For particles >38pm, however, use of a round-bottomed beaker, an efficient stirrer and/or a viscous electrolyte is necessary to overcome the problem of keeping the particles in suspension. The instrument is particularly useful when only small samples are available since low concentrations and very small amounts of material are preferred (Walker et al 1974) . It is possible to obtain a more detailed size distribution by using the method of McCave (1979) to increase the normal 16 data points to 41 by taking three overlapping size distributions on each sample.

COULTER COUNTER PROCEDURE (refer to flowchart, Figure 1) 1. Sample preparation

The dialysed sample (20gms) is treated with H209 to remove organic material, then deflocculated with a filtered solution of sodium hexametaphosphate (Calgon - 5gm to 1 litre distilled water) and sieved at 63um. The <63pi filtrate is put into a 2 litre beaker. Five millilitres of sodium azide solution is added to prevent bacterial growth (lgm to 500mls distilled water) and the suspension made up to 1 litre with distilled water. The sample is agitated using a magnetic stirrer in a beaker fitted with a perspex baffle which prevents th

We have found that if the sample contains a high proportion of heavier silt particles, it is necessary to put the stirrer speed to full for about 30 seconds to get these particles into suspension before reducing it to a speed of 7 or 8 whilst subsampling. Following Shideler's method (1976), an aliqi'ot of the suspension is removed from the bottom of the beaker, a second from the middle and a third from the top to provide a representative subsample. The amount removed by pipette depends on the concentration of the suspension and in our case ranged from 0.3 to >10mls for each aliquot.For samples containing only small quantities of fine silt and clay, a larger aliquot is needed (10mls), whereas for finer sediment an average of 0.5 - lml is satisfactory. N.B. During actual analysis on the Coulter Counter, the aperture tube is positioned in the same place in a round-bottomed, baffled beaker for each count and the suspension is sampled only from this position, not from the top, middle and bottom as in the initial subsampling stage. Changes in the clay size distribution occur with time due to prolonged contact with the electrolyte (Shideler,1976). Because of this problem of rapid deterioration it is practical to make up samples immediately before use or the day before. It takes about 3-4 hours to prepare 27 samples and 7.5 hours to run them. Our samples are run in batches of 9 so that the analyses for each sample on both tubes are obtained within 2 hours of each other. A batch of 9 takes 2.5 - 3.5 hours to run on both tubes.

In order to combine the Coulter Counter <63um data with the >63um data, the proportions of clay:silt:sand were obtained using the pipette method. Sand size distributions are obtained separately by the settling-tube method. We consider that it would 135 be better to directly overlap the Coulter Counter data with the settling-tube data by accurately wetsplitting a sample in two and analysing one half on the settling tube down to 63pm and the other half up to 100um on the Coulter Counter. This in fact is the method recommended by the work manuals for Coulter Counter procedures.

2. Coulter Counter Procedure Once in the laboratory, with the prepared samples plus a calculator, the Coulter Counter must be switched on and allowed to warm up for 10 minutes. In the meantime the filtrate can be made up. a) Electrolyte preparation The filtration system consists of an 8-litre container attached to the filtration unit. The solution in this container is drawn into the unit through the bottom flow pipe by a suction pump. It passes through a 0.45um filter, comes out of the upper pipe and is returned to the container. The solution is made up of 400mls concentrated prefiltered Calgon solution (100gms to 1 litre distilled water) in 8 litres distilled water. The filtration system needs to run for at least half an hour before it is "clean" enough i.e. has a low background count (see Daily Operating Guide,American manual p.i-1 and Figs 2c and d). The maximum background count for the 280um tube is 80 and for the 70um tube,1200. Counts have been obtained as low as 6 for the 280um tube and 420 for the 70fim tube. It is useful to run the filtration system overnight in order to obtain a very clean electrolyte. b) Calibration Once the machine has warmed up, a routine check must be carried out (American manual p.4-1) and the machine calibrated. The calibration need not be done every day. We have found that 136 there is very little drift in the machine if it is in continual use. If operation is discontinuous, it is necessary to calibrate the instrument before use. The calibration procedure is clearly set out in the American manual p.2-1. Coulter Electronics technicians carried out resistance measurements for us. For the 280pm tube we use a latex containing 12.5pm monosize particles and for the 70pm tube we use 8.06pm particles in the calibration procedure. The bottles ot calibration particles are kept in a drawer beneath the Coulter Counter. There are two methods of calibration, one using the built-in meter and one using the population accessory. We use the latter method (p.2-4, American manual). It is essential to note down the calibration and matching switcJi settings so that they are not forgotten. There is a data sheet provided for this purpose (see our example. Table 1). Figure 2g and h illustrate the oscilloscope screen during the calibration procedure. Table 2 gives the calibration settings which we have used and shows that very little instrument drift occurred during continued use. c) Running Procedure Stopcock grease should be used when fitting the aperture tube to the sample stand. If too much grease is smeared on, it collects in globules and may fall to the bottom of the tube thus blocking it. When the tube is fitted, it is placed in a beaker of clean electrolyte which is drawn up into the aperture tube by opening both stopcocks on the sample stand. There should be plenty of clean electrolyte in the reservoir flask. The background count must be checked. The 280pm tube is used for te <63um subsample and the 70pm tube for the <38pm subsample. Put the <63um subsample into the round-bottomed, baffled beaker (designed after Harfield et al, 1971; Fig.4a) making sure that all the sample has been removed from the storage container 137 (a washbottle of filtered electrolyte comes in handy) and fill the beaker with clean electrolyte. When removing one beaker from the sample stand and replacing it with another, we find it useful to put the stirring rod against the aluminium side wall and rest the sample stand beneath it to prevent the sample stand from crashing into the aperture tube. i) Stirring rod Place the round-bottomed beaker on the stand and position the stirring rod so that it is in the centre of the beaker just off the bottom. The position of the stirring rod must be kept constant fjt each sample otherwise noticeable differences occur in the si :e distributions obtained. The running speed must also »-e kept constant for each analysis during the counting time. We have found that for the <63jJm sample, the stirrer needs to be swtched to full to get the particles into suspension and then the speed reduced until there is no surface turbulence while the analysis is being made. As a number of counts are made on each sample, the stirrer speed must be reduced as the level of suspension in the beaker decreases. Figures 3e-g illustrate the differences in size distribution obtained with varying stirrer speeds. ii) Concentration of suspension The concentration of the sample is critical and must be below 5% coincidence index to prevent coincidence errors (see following section on Accuracy). Correct concentration may be obtained by:- 1. obtaining a small initial subsample; 2. wetsplitting using a Y-shaped glass tube resting on a retort stand; 3. allowing half of the suspension to run to waste whilst keeping 138 the stirrer going, then topping the beaker up with clean electrolyte (useful for the 280pm tube). 4. removing the aperture tube and allowing the sample to run to waste through the plastic inner tLbe (useful for the 70um tube) .

Table 3 gives a rough guide to the total counts for various concentrations of coarse and medium to fine silts. We considered an analysis satisfactory if two successive runs were similar although sometimes a third was taken if the tube became blocked.

iii) Counting There are a number of methods for counting and sizing sample on the Coulter Counter. 1. counting total number of particles 2. allowing a certain volume of suspension to be counted e.g. 2mls for the 280um tube and 0.5mls for the 70um tube. 3. counting the passing particles for a predetermined period. We discounted the latter two methods because the concentration of our samples varied widely. It is necessary to watch the oscilloscope screen during accumulation as aberrations on the screen will indicate a blocked tube or outside electronic interference (Fig. 3a). Good runs are indicated by the type of oscilloscope picture given in Figs.2a and b. If a total number of particles is being counted, it is possible to change the preset number whilst the analysis is running. For example, the preset count is 100,000 and it needs to be changed to 75,000. First put the preset to 175,000, then remove the 'l» of the 175,000 to get 075,000. If there is a complete row of zeroes, the analysis will stop. 139 iv) Data readout Once the count is completed, the stopcock should be closed to prevent sample loss. The readout display will give the time taken for the analysis (generally 2-3 minutes for each run), the total number of particles counted, the percentage total volume in each channel and the number of particles counted in each channel. The scope display knob is switched to PULSE AMP when accumulating. To get a readout of the percentage total volume and number of particles counted in each channel,switch the knob tc DIF. For a cumulative readout of these values, switch to CUM. We noted on our worksheet (see Table 4) the total number of particles counted, the concentration, the time taken for each analysis, the differential percentage total volume for each channel and the number of particles counted in channel 5 which was our overlap channel for the two-tube analysis we used (see below).We noted cnly the values for channels 3 to 14 because these were the active channels (N.B. tube is accurate for particles between 2-40% of its size, so for the 280pm tube the size range would be 5.6 to 112pm and for the 70pm tube from 1.4 to 28um). v) Preparation of the <38pm sample. When the round-bottomed beaker has been removed from the sample stand, it is replaced with a beaker of clean electrolyte. Both stopcocks are opened and the aperture is thoroughly flushed to remove heavier silt particles which fall to the bottom of the tube and can become resuspended during later analyses leading to inaccurate counts {Fig.4b). An aperture tube with a foot has been designed by Harfield et al (1978) to deal with this problem (Fig.4c). The outside of the tube is also washed down to prevent contamination between samples. The analysed sample is placed in a labelled tub and the 140 remaining samples in a batch are run. After fitting the 70pm tube,(which was stored in dilute ISOTERGC while not in use), the correct aperture matching switch and size calibration settings are selected. The sample is then sieved a. 38um using a minimum of electrolyte and the <38um fraction put into the cleaned round- bottomed beaker and made up to 4

<20um in size, whereas ours range up to 38 un, so we keep the stirrer going at a gentle speed. The results (percentage total volume, total number of counts and time) are noted at the bottom of the data sheet, underneath the results of the 280um tube.

RESULTS

The method of combining the two sets of data from the the 280pm and 70pm tubes is described on p.68 and illustrated on p.69, British manual. Table 5 is an example from one of our samples. The X-Y recorder is useful for plotting the results of a single tube analysis, but it is inapplicable for 2 tube analyses. Figs 3b,c,d show some sample distributions as they appear on the oscilloscope screen. We have used standard methods of dispersed sediment analysis for two reasons; firstly to facilitate comparison with other studies and secondly because of the difficulty of recreating the environment of deposition. Much of the bottom sediment may reach the sea floor in the form of floccules (Zabawa 1978, Honjo 1978). However there are many problems in trying to disaggregate sediment retrieved by core without breaking up the floes. There are also difficulties in measuring floccules with a Coulter Counter (Treweek and Morgan 1977) because the instrument will measure the particulate nutter within a floe and not the floe as a whole. Several facets of sample preparation which are considered standard in many laboratories are now being questioned e.g. removal of organic matter (Risk 1982). However the philosophy of sample preparation is beyond the scope of this present contribution. From the above description it is obvious that results depend on maintaining standard procedures in sample treatment, subsampling and analysis. 142

ACCURACY The accuracy of a particle size analysis is a function of sampling and of the analysis method. 1. Subsampling Lloyd et al (1978) tested the following methods of subsampling <40um samples a) pipetting from a stirred suspension; b) shaking a suspension and pouring; c) using a Burt sampler whereby compresed air forces a suspension in a cylinder down six capillary tubes. The last two methods were found to be most accurate for the <40um sample. Note, however, that in their tests only a single sample was extracted by pipette. For our <63pm samples the Burt method may be a better method. Shaking and pouring would not be sufficiently accurate because of the heavy silt particles in our samples.

2. Analysis Shideler (1976) found that particle counting techniques yielded slightly coarser size distributions than pipette techniques. This is considered to b« due to a) coincidence, b) particle deviation from spherical shape assumed in pipette methods, c) the truncation of the analysis at the lower limit of analysis in the counting technique. Coincidence errors occur when two or more particles pass thrugh the orifice simultaneously. This leads to loss of counts and recording of bigger particle sizes than reality. This effect is minimised by maintaining coincidence index below 5% (Lloyd et al, 1970). 143

PRECISION Reproducibility depends upon rigidly following strict sampling and analysis procedures. Points to look out for are stirrer speed and pipette level when initially subsampling and stirring and position and speed when counting on the Coulter Counter. We undertook a simple statistical test to determine the within sample and between sample reproducibility. Five subsamples were taken from one sample and five runs were made on each subsample with both the 280pm tube and the 70um tube. The test showed that repeated runs on each sample are reproducible and that runs on each subsample are comparable. However/ it was noted that the standard deviation for analyses on the small tube were higher than those on the large tube.

TROUBLE-SHOOTING 1. Bacterial growth in samples This is a major problem in the preparation of samples for the Coulter Counter. Formalin and sodium azide have been used as bactericides with only limited success. Prepared samples should be stored in a cool place out of sunlight to prevent the formation of and run as soon as possible. Part of the problem with bacterial growths seems to originate from the Calgon solution itself. When a solution is made up, growths in it are immediately seen. Detailed programme planning prior to sample collection is suggested so that samples can be analysed as soon as possible after retrieval. It is also suggested that when cores are halved, they be sprayed with a bactericide such as sodium azide. 2. Blocked aperture tubes This occurs more frequently with the 70pm tube than the 280um tube. Blockage may be due to*- 144

a) concentration of suspension too high b) an inadequately dispersed sample ie. sample containing floes which according to Shideler (1976) nay be formed by a high stirrer speed. c) dust particles d) extraneous particles due to breakdown of the filtering unit - a frequent check on the electrolyte background count is needed to prevent this. P.6-2, American manual, deals with methods of unblocking aperture tubes. 3. Filtration unit The filter in the filtration unit does tend to get blocked after about one month's continuous use and a decrease in the flow of electrolyte through the system is a symptom of this. The filter can be removed and soaked in a dilute detergent eg. 'Milton', for a few days and then rinsed in clean distilled water for another few days after which it may be reused. This cleaning process can be carried out only a few times before the filter must be replaced. 4. Calculators Care must be taken not to use a calculator whilst a count is being made because of electronic interference with the Coulter Counter. 5. Vacuum efficiency Sometimes the vacuum is inefficient and the mercury level does no pass the "elbow" near the coalescing bulb on the sample stand (Fig.1-8, p.1-18, American manual), in which case the instrument will not accumulate counts. The vacuum can be adjusted by turning the vacuum regulator knob en the sample stand, or by decreasing the length of the rubber tubing attached to the vacuum pump. N.B. The vacuum pump must be oiled once a week. 145

6. Trap flask If the trap flask is overfull, waste frUid is drawn into the vacuum pump damaging it. 7. Reservoir flask If the reservoir flask empties, it is not possible to fill the aperture tube. Sometimes the end of the tubing becomes uncovered although there is still clean electrolyte in the flask.

ACKNOWLEDGEMENTS

Work on the Coulter Counter was arranged through J.G.Field. Ann Gedye and Roberta Griffiths provided useful tips. Eddie Hendricks and Reg Linden of Coulter Electronics were always ready with advice and service. Gerald Shideler of the U.S.Geological Survey sent us details of his methodology.

REFERENCES

Harfield,J.G.- Miller,B., Lines,R.W., Godin,T. 1978. Large particle measurement with the Coulter Counter. In: Groves,M, J. (Ed.) . Particle size analysis. 378-394. Honjo,J. 1978. Sedimentation of materials in the Sargasso Sea at a 5367 m deep station. J^ Mar. Res. V36,3:469-492. Lloyd,P.J., Scarlett,B., Sinclair,I., 1970. Effect of particle size and concentration on the response of a Counter Counter. In: Groves and Wyatt-Sargent, Particle Size Analysis, pp.276-292. Lloyd,P.J., Stenhouse,J.I.T., Buxton,R.E. 1978. Comparison of the errors due to sampling and to analysis by Counter Counter. In: Groves,M.J.(Ed.)• Particle size analysis:367-377. McCave,I.N. 1979. Diagnosis of turbidites at sites 386 and 387 by particle-counter size analysis of the silf (2-40 m) fraction. In_: Tucholke,B.E., Vogt,P.R. et al. Initial Reports of the Deep Sea Drilling Project, VXLIII, Washington, (U.S. Government Printing Office):395-405. Risk,M.J. 1982. Effects of organisms on sedimentary models. Episodes v2:4-5 Shideler,G.L. 1976. A comparison of electronic particle counting and pipette techniques in routine mud analysis. Jj_ Sed. Pet. V46,4:1017-1025. 146

Treweek,G.P., Morgan,J.L. 1977. Size distributions of flocculated particlec : application of electronic particle counters. Env. Sci. and Tech. Vll,7:707-714. Walker,P.H., Wodyer,K.O., Hutka,J. 1974. Particle size measurements by Co'Iter Counter of very smail deposits and low suspended sediment concentrations in streams. J^ Sed. Pet. V44(3):673-679. Zabawa,C. 1978. Microstructure of agglomerated suspended sediments in Northern Chesapeake Bay Estuary. Science V202:49-51. FIGURE CAPTIONS

Figure 1. Flow chart of procedures.

Figure 2. Various diagnostic oscilloscope configurations.

Figure 3. Oscilloscope configurations, b) - d) show typical distributions, e) - g) show the effect of stirring speed on replicates of a single sample.

Figure 4. a) Design of round-bottomed baffled beaker. b) and c) show standard and modified aperture tubes with coarse particles accumulating at the bottom. TABLE 1

DIAMETER (u) CHANNEL 70y TUBE 280u TUBE 0.794 1 1.00 2 1.26 3 1.59 4 2.00 5 2.52 6 3.17 7 4.00 8 1 5.04 9 2 6.35 10 3

8.00 11 4 10.08 12 5 12.7 13 6 16.0 14 7 20.2 15 8 25.4 16 9 32.0 10 40.3 11 50.8 12 64.0 13 80.6 14 101.6 15 128.0 16 TABLE 2

CALIBRATION SETTINGS USED

setting for setting for Date 280u tube 70y tube

11/9/80 119.8 153.09

22/10/80 111.1 152.3

TABLE 3

RECOMMENDED TOTAL COUNTS FOR VARIOUS CONCENTRATIONS OF COARSE AND MEDIUM-FINE SILTS

Concentration coarse med-fine

1% 100 low 30 000 2% 30 000 50 000 3% 40 000 50 000 4% 50 000 100 000 5% 50 000 150 000 TABLE 4

PLE 5746 331-333 28On Tube NNEL DIFF.% PARTICLES COUNTED 3 3.7 TOTAL COUNT 100 000 4 4.2 13645 5 5.6 TIME 171.7 SECS (OVERLAP CHANNEL) 6 6.8 7 8.3 CONCENTRATION 2% 8 9.1 INDEX 9 11.8 10 13.6 11 16.7 12 17.6 13 3.2 14 0.2 70M Tube 3 3.9 4 2,4 TOTAL COUNT 63544 5 3.2 6 4.1 TIME 35.0 SECS 7 4.4 8 4.0 CONCENTRATION 3.5% INDEX 9 5.0 10 5.6 11 7.1 12 7.4 122 (OVERLAP CHANNEL) '3 10.0 14 9.5 TABLE 5

Sample No. 5114 161-163

small áiam. small large A vol. A vol. ratio tube X cum. % u tube tube ratio = vol. 0.82

1.26 3 6.1 5.0 131.2 100.0 1.59 4 3.8 3.1 126.2 96.2 2.00 5 3.6 3.0 123.1 93.8 2.52 6 3.8 3.1 120.1 91.5 3.'7 7 4.4 3.6 117.0 89.2 4.00 8 4.8 3.9 113.4 86.4 5.04 9 6.4 5.2 109.5 83.4 6.36 10 3 7.9 5.5 0.70 6.5 104.3 79.5 8.0 11 4 8.9 6.4 9.72 7.3 97.8 74.5 10.08 12 5 9.6 8.5 0.89 7.9 90.5 69.0 12.7 13 6 12.4 10.5 0.85 10.2 82.6 62.9 16.0 14 7 11.7 12.4 1.06 72.4 55.2 20.2 8 12.7 60.0 45.7 25.4 9 13.3 47.3 36.0 32.0 10 12.0 ave * 34.0 25.9 40.3 11 11.6 0.82 22.0 16.8 50.8 12 8.9 10.4 7.9 64.0 13 1.5 1.5 1.1 80.6 14 0 0 0 fif 1. SAMPLE PREPARATION

(''•- — dialyse treat w th o H O add uaigon eve 63u *A ^J 2 z ZLA Ogms original <63u sample sample

A make up to 11. subsample using >| add sodium azid " <•> »^-< in 21. beaker —» magnetic stirrer 7 V" i Ï 'A <63u sample Coulter Counter subsample

2. SAMPLE PROCESSING

make up to 4G0mis. run analysis >j in r/bottomed beaker (see manual) wet sieve 38u A with filtered electrolyte ^ Coulter Counter subsample <38usample

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XV CONSOLIDATED LIST OF PUBLICATIONS OF THE MARINE GEOSCIENCE GROUP Papers Published 1961-1980 Birch,G.F., and Rogers,J. 1973. Nature of the sea floor between Luderitz and Port Elizabeth. S_^ Afr. Shipping News and Fishing Ind. Rev.,28(7):56-65. , and Willis,J.P. 1975. Phosphorite formation on the continental margin of South Africa. Proc. Electron Microsc. Soc. S. Afr.,4:79-80. , Willis,J.P., and Rickard,R.S. 1976. Electron microprobe investigations of pelletal phosphorite off the west coast of South Africa. Proc. Electron Microsc. Soc. S. Afr.,6:91-92. , , and 1976. An electron microprobe study of glauconites from the continental margin off the west coast of South Africa. Mar. Geol.,22:271-283. 1976. Surficial sediments of Saldanha Bay and Langebaan Lagoon. Trans, geol. Soc. S. Afr.,79(3) :293-300. , Rogers,J., Bremner,J.M., and Moir,G.J. 1976. Sedimentation controls on the continental margin of southern Africa. First interdisciplinary Conf. mar, freshwater Res. S. Africa. Port Elizabeth, June, 1976. 1977. Surficial sediments on the continental margin off the west coast of South Africa. Mar. Geol.,23:305-337. 1977. Phosphorites from the Saldanha Bay Region. Trans. R. Soc. S^ Afr.,42(3+4):223-240. 1978. Penecontemporaneous phosphatization by replacement and precipitation mechanisms on the western margin of southern Africa. In: Friedman, G.M. (ed.). Abstr. 10th int. Congr. Sediment., Jerusalem, Israel, 9-14th July. 1:71 1978. The distribution of clay minerals on the continental margin off the west coast of South Africa. Trans, geol. Soc. S^ Afr.,81:23-24. , du Plessis,A., and Willis,J.P. 1978. Offshore and onland geological investigations in the Wilderness Lakes region. Trans, geol. Soc. S. Afr.,81:339-352. 1979. The nature and origin of mixed glauconite/apatite pellets from the continental margin of South Africa. Mar. Geol.,29:313-334. 148

1979. Phosphatic rocks on the western margin of South Africa. J^ sedim. Petrol.,49:93-118. 1979. The association of glauconite and apatite minerals in phosphatic rocks from the South African continental margin. Trans, geol. Soc. S. Afr.,82(l):43-53. 1979. Phosphorite pellets and rock from the western continental margin and adjacent coastal terrace of South Africa. Mar. Geol.,33(1/2);91-117. 1980. A model of penecontemporaneous phosphatization by diagenetic and authigenic mechanisms from the western margin of South Africa. J^ sedim. Petrol. Special SEPM Publ. No.,29:79-100. Bolli,H.M., Rvan,W.B.F., Foresman,J.B., Hottman,W.E., Kagami,H., Longoria,J.F., McKnight,B.K., Melguen,M., Natland,J., Proto-Decima,F., Siesser,W.G. 1975. Initial Core Descriptions (ICD) Deep Sea Drilling Project Leg 40. Scripps Inst. Oceanog.,. La Jo 11a, I49p. Bremner,J.rt., and Willis,J.P. 1975. Glauconite and phosphorites from South West Africa. Proc. Electron Microsc. Soc. S. Afr., 5:127-128. , and Rickard,R.S. 1977. On the formation of phosphorite pellets. Proc. Electron Microsc. Soc. S. Afr., 7:83-84. 1978. Trace element concentrations in authigenic minerals from South West Africa. In: G.M. Friedman (ed.) Abstr. 10th int. Congr. Sediment. Jerusalem, July 9-14, 1978. 1:87-88. 1978. South West African offshore phosphorites. In: Proterozoic- Phosphorites. First int. field Workshop and Seminar, Australia, Aug. 1978:56-58. 1979. Sediment dispersal processes in Algoa Bay. Abstr. 4th nat. oceanogr. Symp., Cape Town, lp. 1980. Physical parameters of the diatomaceous mud belt off South West Africa. Mar. Geol.,34:M67-M76. 1980. Concretionary phosphorite from S.W. Africa. J. geol. Soc. Lond.,137:773-786. Bryan,G.M., and Simpson,E.S.W. 1971. Seismic refraction measurements on the continental shelf between the Orange River and Cape Town. Rep. Inst, geol. Sci. London, 70/16:187-198. Dingle,R.V. 1969. Marine Neocomian octracods from South Africa. Trans. R. Soc. S. Afr.,38:139-164. 149

1969. Upper Senonian ostracods from the coast of Pondoland. Trans. R. Soc. S. Afc.,38:347-385. 1970. Preliminary geological map of part of the eastern Agulhas Bank, South African continental margin. Proc. geol. Soc. Lond.,1663:137-142. 1971. Cytherelloidea gardeni nom.nov. (ostracoda). Trans. R. Soc. S. Afr.,39:~353. 1971. Tertiary sedimentary history of the continental shelf off southern Cape Province, South Africa. Trans, geol. Soc. S^ Afr.,74:173-186. 1971. Some Cretaceous ostracodal assemblages from the Agulhas Bank (South African continental margin). Trans. ÍLl ^c» EL Afr«»39:393-418. , Gentle,R.I., Gerrard,I., and Simpson,E.S.W. 1971. The continental shelf between Cape Town and Cape Agulhas. Rep. Inst, geol. Sci. 70/16:199-209. , and Klinger,H.C. 1971. Significance of Upper Jurassic sediments in the Knysna Gutlier (Cape Province) for timing of the breakup of Gondwanaland. Nature, Lond.232:37-38. , and Gentle,R.I. 1972. Early Tertiary volcanic rocks on the Agulhas Bank, South African continental shelf. Geol. Mag.,109:127-136. , and Klinger,H.C. 1972. The stratigraphy and ostracod fauna of the Upper Jurassic sediments from Brenton, in the Knysna Outlier, Cape Province. Trans. R. Soc. S. Afr.,40:279-298. , and Rogers,J. 1972. Pleistocene palaeogeography of the Agulhas Bank. Trans. R. Soc. S. Afr.,40:155-165. , and 1972. Effects of sea-level changes on the Pleistocene palaeoecology of the Agulhas Bank. In:Van Zinderen Bakker,E.M. (ed.) Palaeoecology of Africa 4:55-58 1973. The geology of the continental shelf between Luderitz and Cape Town, with special reference to Tertiary strata. J. geol. Soc. Lond., 129:337-363. 1973. Post-Palaeozoic stratigraphy of the eastern Agulhas Bank, South African continental margin. Mar. Geol., 15:1- 23. 1973. The distribution and thickness of post-Palaeozoic sediments on the continental margin of southern Africa. Geol. Mag., 110:97-102. 150

1S73. Mesozoic palaeogeography of the southern Cape, South Africa. Palaeogeogr. Palaeoclimat. Palaeoecol., 13:203- 213. 1975. Book Review. Petroleum and the continental shelf of North West . Quart. News Bull, geol. Soc. S. Afr.,18(4);12-13. 1976. Palaeogene ostracods from Uhe continental shelf off Natal, South Africa. Trans. R. Soc. S. Afr.,42:35-79. 1976. A review of the sedimentary history of some post- continental margins of Atlantic-type. In: De Almeida,F.F.M. (ed.) Int. Symp. continental margins of Atlantic type. Ann. Brazil Acad. Sci.,43 (Supplement):67- 80. , and Siesser,W.G. 1976. Geological map of the continental margin between Walvis Bay and Ponto do Ouro. First interdisciplinary Conf. mar, freshwater Res. S. Afr.. Port Elizabeth. Fiche 6 D4. , and Simpson,E.S.W. 1976. The Walvis Ridge, a review. In:Drake,C.L.(ed.) Geodynamics : Progress and Prospects . Am. geophys. Union: Washington.160-176. , and Scrutton,R.A. 1976. Continental margin fault pattern mapped southwest of Iceland. Nature,268:720-2. , Moir.G.J., Bremner,J.M. , and Rogers,J. 1977. Bathymetry of the continental shelf off the Republic of South Africa and South West Africa. Map geol. Surv. S. Afr. mar. Geosc. Ser. 1. , and Siesser,W.G. 1977. Geology of the continental margin between Walvis Bay and Ponta do Ouro. Map geol• Surv. S. Afr. mar. Geosc. Ser. 2. 1977. Tne anatomy of a large submarine slump on a sheared continental margin (SE Africa). Jj_ geol. Soc. Lond.,134:293-310. , Goodlad,S.W., and Martin,A.K. 1978. Bathymetry and stratigraphy jf the northern Natal Valley (SW Indian Ocean): a preliminary account. Mar. Geol.,28:89-106. 1978. South Africa. In:Moullade,K. and Nairn,A.E.M.(eds) Phanerozoic Geology of the World: II. The Mesozoic A. Elsevier:Amsterdam 401-434. 1979. Sedimentary basins and basement structures on the continental margin of southern Africa. Bull. geol. Surv. S. Afr.,63:29-45. 151

, Lord,A.R., and Hendey,O.B. 1979. New sections in the Varswater Formation (Mio-Pliocene) of Langebaan Road, S.K. Cape. Ann. S. Afr. Mus., 78:81-92.

, and Scrutton,R.A. 1979. Sedimentary succession and tectonic histoiy of a marginal plateau (Goban Spur, S.W. of Ireland). .«lar. Geol., 33:45-69. , and Camden-Smith,F. 1979. Acoustic stratigraphy and current-generated bedforms in deep ocean basins off S.E.Africa. Mar. Geol.,33:239-260.

1980. Santonian and Campanian ostracods from a borehole at Richards Bay, Zululand. Ann. S. Afr. Mus.,82:1-70. 1980. Lirge allochthonous sediment masses and their role in the construction of the continental slope and rise off South Western Africa. Mar. Geol.,37:333-354.

1980. Sedimentary basins on the continental margin of southern Africa and an assessment of their hydrocarbon potential. Erdol und Kohle,33:457-463. Du Plessis,A., Scrutton,R.A., Barnaby,A.M., and Simpson,E.S.W. 1972. Shallow structure of the continental margin of southwestern Africa. Mar. Geol.,13:77-89.

, and Simpson,E.S.W. 1974. Magnetic anomalies associated with the southeastern continental margin of South Africa. Mar, geophys. Res., 2:99-110.

1976. The Evolution of the Southeastern Atlantic Ocean - A Review. First interdisciplinary Conf. mar. freshwater Res. S. Afr. Port Elizabeth. Workshop Reports. Fiche.

, and De la Cruz,M.A. 1977. Geophysical investigations in Saldanha Bay. Trans. R. Soc. S. Afr.,42(3+4):285-302.

1977. Seafloor spreading south of the Agulhas Fracture Zone. Nature,270:719-720. Flemming,B.W. 1976. Rocky Bank - evidence for a relict wave-cut platform. Ann. S. Afr. Mus.,71:33-48.

1976. Side-Scan Sonar : a practical guide. Int. hydrogr . Rev.,53(1) :65-92.

1977. Distribution of Hecent sediments in Saldanha Bay and Langebaan Lagoon. Trans. R. Soc. S. Afr.,42:317-34C.

1977. Langebaan Lagoon - A mixed carbonate-siliciclastic tidal environment in a semi-arid climate. Sedim. Geol.,18:61-95.

1977. Depositional history of Saldanha Bay sediments interpreted by scanning electron microscopy. Proc. Electron Microsc. Soc. S. Afr., 7:77-78. 152

1978. Sand transport patterns in the Agulhas Current (South-east African continental margin). In: Friedman, G.h. (ed.) Abstr. 10th int. Congr. Sediment., 1:210-211. Jerusalem, Israel, 9-14th July. 1978. Depositional processes in an equilibrated pocket bay along the west coast of South Africa.In: Friedman, G.M. (ed.) Abstr. 10th int. Congr. Sediment., 1:212-213. Jerusalem, Israel, 9-14th July.

1978. Underwater sand dunes along the southeast African continental margin - observations and implications. Hat. Geol. 26:177-198.

1978. Geological and sedimentological processes. In: Heydorr.,A.E.F. (ed.) , Ecology of the Agulhas Current region: an assessment of biological responses to environmental parameters in the south-west Indian Ocean. Trans. R. Soc. S. Afr.,43:162-167.

1979. Generation and control of sedimentary bedforms in the Agulhas Current (Southeast African Continental Margin). Abstr. 4th nat. oceanogr. Symp. (Cape Town, July 10-13, 1979). S 189.

1979. Application of side-scan sonar techniques for geological mapping of the sea-bed. Abstr. 4th nat. oceanogr. Symp. (Cape Town, July 10-13, 1979). S 189.

1979c. The late Quaternary evolution of Langebaan Lagoon. Abstr. 18th Congr. geol. -,oc. S. Af r. (Port Elizabeth, S. Afr .). Part 1:138-141.

1980. Guide to the Cainozoic deposits of the Saldanha Bay/Langebaan Lagoon area. CSIR Report SEA 8002, 69p.

1980. Sand transport and bedform patterns on the continental shelf between Durban and Port Elizabeth (Southeast African Continental Margin). Sedim. Geol.,26:179-205.

1980. Factors controlling shelf sediment dispersal along the southeast African continental margin. Abstr. 26th int. geol. Congr. (Paris, July 7-17, 1980). II :469.

Fuller,A.0. 1961. Size distribution characteristics of shallow marine sands from the Cape of Good Hope, South Africa. J. sedim. Petrol.,31:256-261.

1962. Systematic fractionation of sand in the shallow marine and beach environment off the South African coast. J. sedim. Petrol.,32:602-606.

, and Lloyd,A. 1964. Lognormal components in natural polymineralic associations. Nature, 201:131. 153

, and Lamming,P.J. 1967. The hydraulic equivalence of quartz and zircon in coastal deposits from the south western of the Cape Province, and its application as an environmental indicator. S_^ Af r. J. Sci. ,63:521-526. Glass,J.G.K., and du Plessis,A. 1976. The bathymetry of False Bay as an indicator of sea floor geology. First interdisciplinary Conf. mar. freshwater Res. S. Afr• Port Elizabeth, Fiche 6DS-E6, 1B6-E2. Klinger,H.C., Kennedy,W.J., and Siesser,W.G. 1976. Yabeiceras (Coniacian ammonite) from the Alphard Group oft the southern Cape Coast. Ann. S. Afr. Mus.,69:161-168.

Leyden,R.f Ewing,M., and Simpson,E.S.W. 1971. Geophysical reconnaissance on the African shelf: I. Cape Town to East London.Bull. Am. Ass. Petrol. Geol., 55:651-657. Lloyd,A.T., and Fuller,A.0. 1965. Glauconite from shallow marine sediments off the South African coast. S. Afr. J. Sci.,61:444-448. Ludwig,W.j., Nafe,J.E., Simpson,E.S.W., and Sacks,S. 1968. Seismic refraction measurements on the southeast African continental margin. Jj_ geophys. Res., 73: 3707-3719. Mallory,J.M., Simpson,E.S.W., and Forder,E. 1964. Bathymetric chart of South African Oceanic Areas. Map Studio Productions: Johannesburg. Marchant,J.W., and Flemming,B.W. 1978. Granite erratics in a sandstone boulder-beach in False Bay - independent evidence for a marine transgression. Trans, geol. Soc. S. Afr.,81:210-221. McArthur,J.M., Coleman,M.L., and Bremner,J.M. 1980. Carbon and oxygen isotopic composition of structural carbonate in sedimentary francolite. J_^ geol. Soc. Lond. , 137:669-673. Needham,H.D., 1962. Ice-rafted rocks from the Atlantic Ocean off the coast of the Cape of Good Hope. Deep-Sea Res.,9:475- 486. Parker,R.J., and Siesser,W.G. 1972. Petrology and origin of some phosphorites from the South African continental margin. J. sedim.. Petrol.,42:4 34-440. Rogers,J., Summerhayes,C.P., Dingle,R.V., Birch,G.F., Bremner,J.M., and Simpson,E.S.W. 1972. Distribution of minerals on the seabed around South Africa and problems in their exploration and eventual exploitation. ECOR Symposium on "The Ocean's Challenge to S.A. Engineers", Stellenbosch, CTS.I.R. S71, 1971. 154

, and Tankard,A.J. 1974. Surface textures of some quartz grains from the west coast of Southern Africa. Proc. Electron Microsc. Soc. S. Afr•,4:55-56. Scrutton,R.A., du Plessis,A., Barnaby,A.M., and Simpson,E.S.W. 1975. Contrasting structures and origins of the western and south eastern continental margins of southern Africa. Proc. 3rd Gondwana Symposium, Canberra, 1973. , , , and 1975. Shallow structure of the continental margin of South Western Africa. Ann, geol. Surv. S. Afr., 9:129-133. , and Dingle,H.V. 1976. Observations on the processes of sediment basin formation on the margins of southern Africa. Tectonophysics,36:143-56. Siesser ,Vv.G. 1970. Carbonate components and mineralogy of the South African coastal limestones, and limestones of the Agulhas Bank. Trans. geol. Soc . S. Afr.,73:49-63. , and Rogers,J. 1971. An investigation of the suitability of four methods used in routine carbonate analysis of marine sediments. Deep-Sea Res.,18:135-139. 1971. Mineralogy and diagenesis of some South African coastal and marine carbonates. Mar. Geol.,10:15-38. 1972. Environmental discrimination of Tertiary limestones by conventional and electron microscopic methods. Trans, geol. Soc. S. Afr.,75:295-298. 1972. Petrology of the South African Coastal limestones. Trans, geol . Soc. S^. Afr. ,75:177-185. 1972. Limestone lithofacies £rom the South African continental margin. Sedim. Geol.,8:83-112. 1972. Dolostone frcm the South African continental slope. J. sedim. Petrol.,42:694-699. 1972. Relict algal nodules (Rhodolites) from the South African continental shelf. J^ Geol. ,8P:611-616. 1972. Abundance and distribution of carbonate constituents in some South African coastal and offshore sediments. Trans. R. Soc. S. Afr.,40:261-278. 1972. Carbonate mineralogy of bryozoans and other selected South African organisms. S_^ Afr. J. Sci. ,68:71-74. 1973. Ca/Mg and Sr/Ca ratios of some South African coastal and offshore carbonate sediments, bull. Am. Ass. Petrol. Geol., 57:930-932. 155

- 1973. Stratigraphic and palaeoclimatic analysis of continental slope sediment cores. Abstr. S. A£r. nat. oceanogr. Symp., 30-31.

- 1973. Diagenetically formed ooids and intraclasts in South African calcretes. Sediment., 20:539-551.

- 1974. Micropalaeontological techniques used in Pleistocene climatology. S^_ Afr• archaeol. Bull. Goodwin Series 2(Sept):29-36.

-, Scrutton,R.A., and Simpson,E.S.W. 1974. Atlantic and Indian Ocean Margins of Southern Africa. In: Burke,C.A. and Drake,C.L.(eds.). The Geology of Continental Margins, Springer-Verlag, New York.

- 1974. Relict and Recent beachrock from Southern Africa. Bull, geol. Soc. Am., 85:1849-1854. - 1975. Dolostone at Saldanha Bay: evidence for Pleistocene desiccation. Trans, geol. Soc. S. Afr.,78:361-365.

- 1976. Calcareous nannofossils in Pleistocene sediment cores from the South African continental slope. Trans. R. Soc. S_L Afr.,42:107-147.

- 1976. Au«-higenic minerals in outer continental margin sediments. First interdisciplinary Conf. mar, freshwater Res. S. Afr. Port Elizabeth. Fiche 6E7-G2.

- 1976. Native copper in DSDP sediment cores firom the Angola Basin. Nature,263:308-309. -, and Rogers,J. 1976. Authigenic pyrite and gypsum in South West African continental-slope sediments. Sedimentology,23:567-577.

- 1977. Late Eocene age of marine sediments at Bogenfels, South West Africa, based on calcareous nanrofossils. In: Papers on Biostratigraphic Research, Bull, geol. Surv. S. Afr., 60:72-74.

- 1977. Chemical composition of calcareous nannofossils. S. Afr-. J^ Sci.,73:283-285.

- 1977. Calcareous nannofossils as age and palaeoenvironmental indicators. Proc. Electron Microsc. Soc. S. Afr., 7:81-82.

-, and Orchiston,D.W. 1978. Micropalaeontological raassessment of the age of Pithecanthropus nandible C from Sangiran, Indonesia. In: Bartstra,G.H. and Casparie,W.A.(eds.). Modern Quaternary Research in Southeast Asia 4. Biol.-archaeol. Inst. Rijksuniversiteit, Groningen, Holland. 156

1978. Age of phosphorites on the South African continental margin. Mar. Geol.,26:M17-M28. 1978. Aridification of the Namib Desert: evidence from oceanic cores. In: Van Zinderen Bakker,E.M.(ed.). Antarctic Glacial History and World Palaeoenvironments. Balkema : RotterJam. 105-113. 1979. Native copper in DSDP Leg 40 sediments. In: Bolli,H.M., Ryan,W.B.F. et al. Init. Reps Deep Sea Drilling Project,XL., Washington (U.S. Govt. Print. Off.):761-765. 1978. Leg 40 results in relation to continental shelf and onshore geology. In: Bolli,H.M., Ryan,W*B.F. et al. Init. Reps Deep Sea Drilling Project, XL. Washington (U.S. Govt. Print7~0Tf.):965-979. , and Bremner,J.M. 1978. X-ray mineralogy of cores from Leg 40, DSDP. In: Bolli,H.H., Ryan,W.B.F. et al. Init. Reps Deep Sea Drilling Project, XL. Washington (U.S. Govt. PrTnt. Off.):541-548. Simpson,E.S.W. 1964. A new world of marine science awaits research. Optima,1964:68-75. 1964. South African research in marine geology. S_^ Afr. J. Sci.,61:51-54. , and Heydorn,A.E.F. 1965. Vema . Nature, 207:249-251 1965. New research vessel for the University of Cape Town. Sci. S^ Afr.,2:436-437. 1966. Die geologie van die vastelandsplat. Tegnikon, 15:168-176. 1967. Ocean wealth on the shelf. Million,1:45-57. 1967. Report on the UNESCO/IUGS Symposium on Continental Drift, Montevideo, 1967. IUGS Geological Newsletter, 1967:39-47. 1968. Marine geology : progress and problems. Proc. geol. Soc. S^ Afr.,71:97-111. , and du Plessis,A. 1968. Bathymetric, magnetic and gravity data from the continental marcMn of south-western Africa. Can. J^ Earth Sci.,5:1119-1123. , and Forder,E. 1968. The Cape submarine canyon. Fish. Bull. S. Afr.,5:35-37. 157

, and 1969. Southeast Atlantic and Southwest Indian , Chart 123 : A, Bathymetry; B, Bathymetric Data Tracks; C, Geophysical Data TracKs. Third Edition (revised Dec. 1969). Dept. Geol., Univ. Cape Town.

, Du Plessis, A., and Forder,E. 1970. Bathymetric and magnetic traverse measurements in False Bay and west of the . Trans. R. Soc. S. Afr.,39:113-116.

1971. The geology of the south-west African continental margin : a review. Rep. Inst, geol. Sci. London, 70/16:153-170.

and Dingle,R.V. 1973. Offshore sedimentary basins on the south-eastern continental margin of South Africa. In: Slant,G.(Ed.). Sedimentary Basins of the African coasts. Ass. Afr. geol. Surv., Paris:63-68.

Schlich,R. et al. 1974. Init. Reps Deep Sea Drilling Project, XXV, Washington. 1. IntroductionT3r2TI

1974. Regional aspects of deep sea drilling in the Western Indian Ocean, Leg 25, DSDP Init. Reps Deep Sea Drilling Project., XXV:743-759.

1974. Site 239. Init .Reps Deep Sea Drilling Project, XXV:25-63. 1974. Site 240. Init. Reps Deep Sea Drilling Project, XXV:65-85.

1974. Site 241. Init. Reps Deep Sea Drilling Project, XXV:87-138.

1974. Site 242. Init. Reps Deep Sea Drilling Project, XXV:139-176.

1974. Sites 243 and 244. Init. Reps Deep Sea Drilling Project, XXV:177-186. 1974. Site 245. Init. Reps Deep Sea Drilling Project, XXV:187-236.

1974. Sites 246 and 247. Init. Reps Deep Sea Drilling Project, XXV:237-257. 1974. Site 248. Init. Reps Deep Sea Drilling Project, XXV:259-286.

1974. Site 249. Init. Reps Deep Sea Drilling Project, XXV:287-346. Summerhayes,C.P., Birch,G.F., Rogers,J., and Dingle,R.V. 1973. Phosphate in sediments off Southwestern Africa. Nature,243:509-511. 158

1973. Distribution, origin and economic potential of phosphatic sediments from the Agulhas Bank, South Africa. Trans, geol. Soc. S. Afr.,76:271-277.

Wefer,G., and Flemming,B.W. 1976. Submarine Abrasion des Geschiebemergels vor Bokniseck (Westl. Ostsee). Meyniana,28:87-94.

Papers Published in 1981 and 1982

Birch,G.F. 1981. Nearshore Quaternary sedimentation off the south coast of South Africa (Cape Town to Port Elizabeth). Bull, geol. Surv. S. Afr., 67.

, 1981. The bathymetry and geomorphology of the continental shelf and upper slope between Durban and Port St Johns. Ann, geol. Surv. S. Afr., 15:55-62.

, 1981. Variations in the clay mineralogy of river samples. Ann, geol. Surv. S. Afr., 15:25-32.

Bremner,J.M. 1981. Shelf morphology and surficial sediment off Central and Northern South West Africa (Namibia). Geo-Marine Letters, 1:91-96.

, and Rogers,J. 1981. Major lithofacies of the Namibian continental margin. In: Suess,E. and Thiede,J. (eds) . Proc. Conf. advanced Res. Inst. Coastal Upwelling: Its Sediment Record. Sept. 1-4, Vila Moura, Portugal.

Coetzee,J.A., and Rogers,J. 1982. Palynological and lithological evidence for the Miocene palaeoenvironment in the Saldanha region (South Africa). Palaeogeography, Palaeoclimatology, and Palaeoecology. 39:71-85.

Denbigh,P.N., and Flemming,B.W. 1982. Range prediction and calibration in side scan sonar. In:Russell-Cargill,W.G.A. (ed.). Recent developments in side scan sonar techniques. Central Acoustics Laboratory University of Cape Town: Cape Town. 81-100.

Dingle,R.V. 1982. Continental margin subsidence: a comparison between the east and west coast of Africa. In: Scrutton,R.A. (ed.). Dynamic . of passive continental margins. Am. geophys. Un., 6: 59-71.

1982. Some aspects of Cret^eous ostracod biostratigraphy of South Africa and relationships with other Gondwanide localities. Cret. Res., 3: 1-23.

, Megson,J.B. & Scrutton,R.A. 1982. Acoustic stratigraphy of the sedimentary succession west of Porcupine Bank, NE Atlantic Ocean: a preliminary account. Mar. Geol., 47:17- 35. 159

Flemming,B.W. 1981. Factors controlling shelf sediment dispersal along the southeast African continental margin, hat. Geol. 42:259-277. , 1981. Richards Bay ocean outfall. Marine geological site investigations during the Meiring Naude cruise 81-01. CSIR Rep. C/SEA 8148. , 1981. Ocean outfall studies at Saldanha, Report No. 2. Marine geological site investigations: North Head to Cape Columbine (February 1981). CSIR Rep. C/SEA 8149. , 1982. A historical introduction to underwater acoustics with special reference to echo sounding, sub-bottom profiling and side scan sonar. In: Russell-Cargill,W.G.A. (ed.) Recent developments in side scan sonar techniques. Central Acoustics Laboratory University of: Cape Town: Cape Town. 3-9. , 1982. Causes and effects of sonograph distortion and some graphical methods for their manual correction. In: Russell-Cargill,W.G.A. (ed.). Recent developments in side scan sonar techniques. Central Acoustics Laboratory University of Cape Town: Cape Town. 103-138. , 1982. Dynamics of large transverse bedforms on the southeast African continental shelf. Abstr. 11th int. Congr. Sediment., 22-27 Aug., Hamilton, Ontario. 73. , 1982. Sediment mixing: its natural occurrence and textural expression. Abstr. 11th int. Congr. Sediment., 22-27 Aug., Hamilton, Ontario. 81. , Martin,A.K., and Engelbrecht,J. 1982. Geophysical surveys for marine engineering projects and environmental data acquisition. Abstr. 3rd Symp. sedim. Div. geol. Soc. S. Afr., 13-14 Sept.«Johannesburg. 53-58. Goodlad,S.W., Martin,A.K., and Hartnady,C.J.H. 1982. Mesozoic magnetic anomalies in the southern Natal valley. Nature, 295:686-688. Gurney,J.J., Walker,C.S.H., Prinsloo,K., Borchers,R., and Flemming,B.W. 1982. Diamond recoveries from the surf zone of the Namaqualand coast near the Olifants River. Abstr. 3rd Symp. sedim. Div. qeol. Soc. S. Afr., 13-14 Sept., Johannesburg. 8~Ï-Ó7. Martin,A.K. 1981. The influence of the Agulhas Current on the physiographic development of the northernmost Natal Valley (S.W. Indian Ocean). Mar. Geol., 259-276. , 1981. Evolution of the Agulhas Current and its palaeoecological implications. S. Afr. J. Scl. 77:547-554. 160

, Hartnady ,C J.H., and Good lad ,S.Vi. 1981. A revised fit of South America and South Central Africa. Earth planet. Sci. Lett., 54:293-305. "

, Goodlad,S.W., and Salmon,D.A. J9R2. Sedimentary basin infill in the northernmost Natal Valley, hiatus development and Agulhas Current palaeo-oceanography. J. geol. Soc. Lond., 139:183-261. Rogers,J. 1982. Lithostratigraphy of Cenozoic sediments between Cape Town and Eland's Bay. In:Coetzee,J.A. and Van Zinderen Bakker,E.N. (eds). Palaeoecol. Afr., 15.

, 1982. Geological history of the Western Cape's coastal lowlands.In: Moll,E. (ed.). Proc. Symp. coastal Lowlands **._ Cape, 19-20 Mar., Bellville. 7-8.

Theron,J.N., Du Plessis,A., and Rogers,J. 1981. Depositional history of the bot River estuary. Proc. Workshop Hes. Cape Estuaries. 23 April, Stellenbosch. CSIR Rep. T/SLA 8111: 109-113.